Note (September 2026): IBM no longer offers face recognition or visual identification through Watson; it discontinued Watson Visual Recognition in 2021 and exited general purpose facial recognition in June 2020. The IBM section below has been corrected.
Four widely used APIs every web developer benefits from knowing are IBM Watson Natural Language Understanding (text analysis), the Amazon S3 API (object storage), the Mailchimp Marketing API (email marketing) and the Twilio SendGrid v3 Web API (transactional email). All four are REST APIs that are still available as of September 2026, so learning them covers AI text analysis, file storage and email delivery.
Key Takeaways
- IBM Watson Natural Language Understanding extracts entities, keywords, relations, sentiment and categories from text; IBM’s image and face recognition services are gone.
- The Amazon S3 API (version 2006-03-01) is a de facto standard for object storage, and several other providers offer S3-compatible APIs.
- Mailchimp has been owned by Intuit since November 1, 2021, and its Mandrill product is now called Mailchimp Transactional.
- SendGrid has been part of Twilio since February 1, 2019, and its v3 Web API handles both transactional email and marketing campaigns.
- Prices and free tiers change often, so check each provider’s official pricing page before you build.
When it comes to APIs there are many thousands of public application programming interfaces (the ProgrammableWeb directory had catalogued over 19,000 open APIs at its peak before it closed on February 3, 2023) provided by numerous companies which are aimed at helping developers save time and directly solve problems. While many developers like to experiment and try new APIs the experience is met with a learning curve.
Top 4 APIs Doing Developer Experience Really Well
That said it is always good to have a short list of highly useful APIs that you can implement just about anywhere. We’ve narrowed down the 14,000 to 4 which are the bare essential that most developers should master.
1] IBM Watson’s AI API
IBM’s Watson is best known for beating many Jeopardy champions using state of the art artificial intelligence developed by IBM. IBM now offers its language technology to developers as a cloud API, most notably Watson Natural Language Understanding, while its newer generative AI work sits in the watsonx platform introduced on May 9, 2023.
Called NLP or Natural Language Processing, it provides developers access to their research directly via RESTful calls. IBM acquired AlchemyAPI in March 2015 and folded its technology into the Watson product line. Image analysis is no longer part of the offering: IBM discontinued Watson Visual Recognition, supporting existing instances only until 1 December 2021.
Among the tool-set is the ability for developers to identify keywords in text and extract relationships between other forms of data derived from the text. It can also score sentiment and emotion and sort content into categories. Face recognition is not available: in June 2020 IBM said it no longer offers general purpose facial recognition or analysis software. So, if you’re looking to add intelligence to your app, then IBM Watson’s API is the best way to do it.
2] The Amazon S3 API
Once an app grows past its prototype phase, it will require storage options which exceed what your machine may be able to handle.
A popular app may easily expand past the space offered by dedicated drives and even shared hosting. If you use cloud hosting it’s hard to get permanent storage.
If you want the system to scale reliably, you need to use object storage like the one offered by Amazon S3. Amazon S3 stores data as objects inside buckets, and it provides a number of handy features, including uploads directly to a storage bucket, object versioning (switched off by default) and replication.
All of this can be easily accessed via Amazon’s S3 API. So, mastering this one API can help solve many of your storage woes.
3] The Popular MailChimp API
If you happen to have a large team sending marketing emails isn’t necessarily technical, but it’s always a good idea to allow marketers to choose their own set of tools.
Fortunately, Mailchimp, owned by Intuit since November 1, 2021, is one of the best-known email marketing platforms, which is why knowing how the Mailchimp Marketing API works is useful. Almost every web application can benefit from the MailChimp API, the same goes for if you are using API development tools to create your own API.
4] Twilio SendGrid
Not all emails are equal, and that’s why a transactional email is very different from a regular marketing email. These transactional emails are usually one email sent with some vital information.
The message is targeted, like passwords, social media notification, password resets and account confirmations amongst others.
The SendGrid v3 Web API, run by Twilio since it completed its acquisition of SendGrid on February 1, 2019, puts transactional email capabilities in the hands of developers. However, the offering has since expanded beyond just real-time email delivery to adding marketing features and triggered responses.
All apps can use this API since most will need to send a notification alerting account holders or members of a transaction.
One well-known alternative to SendGrid is Mailchimp’s own Mandrill. Mandrill has since been renamed Mailchimp Transactional. It is far from SendGrid’s only competitor, and which of the two is easier to learn depends on the team.
Conclusion
The above four mentioned APIs would help you improve your app’s ability to communicate and perform otherwise mundane tasks while you can focus on the application’s core.
Obviously, you may not have to use all four APIs in one application or all at the same time but being familiar with their functionality can help you take applications you develop a long way.
The best way to get started is to write a few lines of code, read the API’s documentation and explore their capabilities in the context of your app.
What Is an API and Why These Four Matter
An API (application programming interface) is a defined way for one program to request data or actions from another. All four APIs in this guide are REST APIs: an app sends an HTTP request to the provider and gets a structured response back. Amazon’s S3 documentation notes that its REST API uses standard HTTP headers and status codes, so ordinary HTTP toolkits work with it. For background, see our explainer on why APIs are the building blocks of modern application development.
Together, the four cover jobs almost every web application needs: understanding text, storing files, emailing an audience and sending account emails.
How the Four APIs Compare
| API | Owner | Main job | Status as of September 2026 |
|---|---|---|---|
| Watson Natural Language Understanding | IBM | Text analysis: entities, keywords, relations, sentiment, emotion, categories, concepts | Available; Visual Recognition discontinued in 2021 |
| Amazon S3 API | Amazon Web Services | Object storage in buckets | Available; current API version 2006-03-01 |
| Mailchimp Marketing API | Intuit (Mailchimp) | Marketing campaigns powered by data from your app | Available; Mandrill now Mailchimp Transactional |
| SendGrid v3 Web API | Twilio | Transactional email, marketing campaigns, contacts, email statistics | Available |
What Has Changed Since This List Was First Published
IBM Watson
IBM Watson won fame in 2011, when it beat Jeopardy! champions Ken Jennings and Brad Rutter. The developer API most closely tied to that language work today is Watson Natural Language Understanding. According to IBM Cloud documentation, it analyzes text, HTML or public URLs and returns entities, keywords, relations, sentiment, emotion, categories (a five-level hierarchy), concepts, metadata, semantic roles and syntax.
Several other Watson services have been retired. IBM’s Watson developer SDK notes list Visual Recognition, Compare and Comply and Personality Insights as discontinued in 2021, and Natural Language Classifier as ceasing operations on 8 August 2022. For generative AI, IBM introduced the watsonx platform on May 9, 2023, made up of watsonx.ai, watsonx.data and watsonx.governance.
Amazon S3
Amazon Simple Storage Service (Amazon S3) launched in the United States on March 14, 2006. A single object can be up to 5 TB, and objects larger than 5 GB need multipart upload. Amazon S3 offers several storage classes with different durability, availability and performance levels, including S3 Express One Zone for latency-sensitive workloads.
The S3 API has become a de facto standard: services such as Backblaze B2 and Wasabi and open-source software such as MinIO offer S3-compatible APIs, so skills learned on S3 carry over. AWS also documents newer S3 features, including S3 Tables for Apache Iceberg tables and S3 Vectors for vector similarity search. Amazon recommends the REST API or the AWS SDKs, because SOAP support is deprecated. Our guide to AWS Cloud Practitioner certification training covers the wider AWS platform.
Mailchimp
Mailchimp was founded in 2001 and changed its styling from MailChimp to Mailchimp in 2018. Its developer site lists three products: the Mailchimp Marketing API, Mailchimp Transactional (formerly Mandrill, sending through the API or SMTP) and Mailchimp Open Commerce. As of September 2026, Mailchimp’s developer page says new Transactional users can get up to 500 free email sends to their verified domain; other pricing changes, so check Mailchimp’s own pricing page.
Twilio SendGrid
SendGrid was founded in 2009 and became Twilio SendGrid after Twilio’s $2 billion acquisition, announced in October 2018 and completed on February 1, 2019. According to Twilio’s documentation, the SendGrid v3 Web API provides a REST interface to send email at scale, and developers use it to send transactional emails, create marketing campaigns, manage contacts and monitor email statistics. Our guide to email marketing strategies covers the campaign side.
How to Start Learning Each API
- Create a developer account with the provider (IBM Cloud, AWS, Mailchimp or Twilio SendGrid) and generate an API key or credentials.
- Store keys safely in environment variables or a secrets manager, never in front-end code or a public repository.
- Read the official reference and make one simple call first, such as analyzing a sentence, uploading one object or sending one test email.
- Use the official SDK for your language once the raw HTTP call works.
- Test with sample data: a tool like our random JSON data generator helps you build test payloads, and a JWT decoder helps when an API uses JSON Web Tokens.
- Watch usage and costs: all four providers charge for usage or plans beyond any free allowance, and prices change, so check current pricing pages.
Common Mistakes to Avoid
- Relying on retired services: older tutorials often reference Watson Visual Recognition or AlchemyAPI, which no longer exist as products.
- Leaving storage open by accident: review S3 bucket access settings before storing user files.
- Mixing marketing and transactional email: password resets and receipts belong on a transactional service such as SendGrid or Mailchimp Transactional, not in a newsletter tool.
- Hard-coding prices or limits: free tiers and plan limits change, so read them from the provider’s current pages.
Frequently Asked Questions
Is IBM Watson’s API still available?
Yes, in part. As of September 2026, IBM still documents Watson Natural Language Understanding as a cloud API for text analysis. Watson Visual Recognition, Personality Insights, Compare and Comply and Natural Language Classifier have been discontinued, and IBM’s generative AI tools now sit in watsonx.ai.
Is the Amazon S3 API free to use?
The Amazon S3 API is the interface to a paid AWS service, so storage and usage are billed by AWS. Prices vary by region and storage class and change over time, so check the current Amazon S3 pricing page.
What happened to Mandrill?
Mandrill, Mailchimp’s transactional email service, was integrated into Mailchimp as a paid add-on in February 2016 and later renamed Mailchimp Transactional. It sends event-driven emails through an API or SMTP.
Who owns SendGrid now?
Twilio owns SendGrid. Twilio completed its $2 billion acquisition on February 1, 2019, and the service is now called Twilio SendGrid.
What is the difference between the Mailchimp API and the SendGrid API?
The Mailchimp Marketing API powers marketing campaigns using data from your app, while the SendGrid v3 Web API is built for sending email at scale, including transactional messages such as password resets, and also supports marketing campaigns. Mailchimp’s transactional equivalent is Mailchimp Transactional.
