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LAST.FM : http://www.last.fm/api Not an elegant/clean API, but the results are amazing. I've build some really fun personal apps with it.

Twitter's streaming APIs are pretty fun:

https://dev.twitter.com/docs/streaming-apis

http://www.feedsapi.com

http://www.sendgrid.com

Did sendgrid redo their API or something? I developed with that about a year ago and remember thinking it was among the worst APIs (though not THE worst) I'd ever had to work with.

well, I've worked with their SMTP API only and never encountered a problem.

I've never had a problem with SendGrid, absolutely love their SMTP and Parse APIs!

Check out the FullContact API http://www.fullcontact.com/developer/.

Stripe.com

http://imdbapi.com/

OpenTok for video chat!

Ask HN: Favorite API