Ask HN: What are your favorite free APIs?

Hi HN -

What are your favorite, free APIs? They can be your favorite for many reasons: making you more efficient, stimulating intellectual curiosity, or simply just for being beautifully designed.

I think it'd be cool to get a list here so that we all can explore the gems our peers have found in their travels around the internet :)

Thanks, Mike

Erlang's OTP API is pretty much the best. Python's logging is nice, too.