Simple math library for the web

I'm looking for a math JavaScript API to show formulas on a web site and allow users to input formulas too (for instance, using a <textarea/>).

The API should be able to parse text strings, such as 3x^2+2 or sqrt(x/(x+5)) building "automagically" fractional layouts, integral symbols and so on.

Thanks.

Take a look at MathJax. It's used at various sites, such as math.stackexchange.com and www.mathoverflow.org.