[bbatsell]

MathJax:

http://www.mathjax.org/

[ReidZB]

This is also what the math-related Stack Exchange sites use. (Ex: Math SE, Crypto SE, Physics SE, ...). For example, see the home page of the Math SE: http://math.stackexchange.com/

[bajsejohannes]

Tip: Wikipedia has experimental support for MathJax, making most equations look way better. (it's in user preferences)