Easy question

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