(via MetaFilter)
PlanetMath.org is a (well, I’ll let them say it) “free, collaborative, online mathematics encyclopedia. The stress is on peer review, rigour, openness, pedagogy, real-time content, interlinked content, and community-drivenness.”
Great reference site.
But don’t forget about Mathword, hosted by Wolfram Research, the Mathematica folks.
If you don’t find it on one of these sites, it ain’t math!