For LaTeX, we have the Font Catalogue which is a good resource for fonts available through LaTeX.
There doesn't appear to be a similar resource for Xe(La)TeX. So let's say I'm bored of computer modern and I want to try out some new fonts in XeLaTeX. Where can I find out what's available? Ideally I'd like to find what fonts have a math alphabet, at least some common ligatures (ff, ffi etc) and old style figures…