I quite like how the Mathematics Stackexchange site's LaTeX looks, and I know that they have a LaTeX functionality by running MathJax on the site. What fonts/packages are available to achieve this look?
As MathJax renders the output locally, the visual appearance is system-dependent. You can see what fonts are being used by right-clicking on the MathJax and choosing 'About MathJax'. For example, in my case I get 'using local STIX fonts' (amongst other things), which makes sense as I have these installed in my system tree. On the other hand, tohecz reports 'otf fonts', presumably Computer Modern or Latin Modern math fonts.
Checking the MathJax font help, it says
To use STIX, I would probably favour using the
although to be honest the XITS bundle is probably a better bet at present.