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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. This thread is only meant for voting on helpful math resources. Thank you for your help!

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!