Many stylesheets contain a list of fonts, in order of descending preference. Just make sure that the last font is guaranteed to be available, Verdana for example.
body { font-family: Helvetica, Verdana; }
If you really want to offer a list of fonts to your users, it doesn't sound pleasant. You need to use flash, and then retrieve the list from here using javascript. This link looks useful: http://nevermore.pri.ee/2007/03/14/another-better-font-detection-using-javascript-flash/
Hope that helps.
Cheers, Chris
