A programmer may use API (application programming interface) in order to interact with some other system. At this post there will be a list of services with APIs and also a link for each API and a lessons (from Code Academy) of how to use it. List of APIs:

1. Youtube – Video sharing platform. The API will Searching for Videos.
2. SoundCloud – A streaming remixes and covers for songs.
3. Box – Online cloud storage.
4. GitHub – A version management for code writing.
5. Apigee – API management platform.
6. WePay – Online payment service (only in the US), Ruby , Python.
7. Mandrill – Email delivery API from MailChimp.
8. NPR – National public Radio.
9. Dwolla – online payment system and mobile payments network.
10. Evernote – Online cloud based note, storage and archive.
11. NHTSA – National Highway Traffic Safety Administration’s API.
12. Twitter – API to Twitter micro-blog.
13. Sendgrid – Cloud-based email infrastructure and delivery engine, with integration via SMTP and HTTP API, also offering analytics and newsletter functionality.

Based on: https://www.codecademy.com/apis