Well, everybody - at least on this site - will know WolframAlpha, right?!? But what are all the other, really helpful math resources out there on the web?

Please don't only post a link but a short description of what it does and why it is helpful.

Please only one resource per answer and let the votes decide which are the best!

Note: Please don't (mis-)use this tread as a discussion-plattform for the shortcomings of WolframAlpha any more - I created a new thread for that: http://mathoverflow.net/questions/3022/shortcomings-in-wolframalpha

This thread is only meant to vote on helpful math resources!

Thank you for your help!