Most helpful math resources on the web

I occasionally find mathoverflow.net rather helpful.

In particular, there's a good list of answers to your specific question here.

I use http://arxiv.org/ all the time.

Researchers post their articles here, so it is a great way to see if anyone have already a proof or an idea on something. Some people regularly access it through

• A different start page at http://front.math.ucdavis.edu/
• A SPIRES search engine at http://www.slac.stanford.edu/spires/

http://www.wikipedia.org

I have learned a lot of mathematics while reading Wikipedia. Allowing a wide audience to contribute to articles seems to work out well in the case of mathematics.