What are 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:** An old version of this post mentioned `WolframAlpha`. Please direct the discussion of its shortcomings to [the new thread][1]. This thread is only meant for voting on helpful math resources.
Thank you for your help!


  [1]: http://mathoverflow.net/questions/3022/shortcomings-in-wolframalpha