Yeah, what Hellhound said.
Morpheus is the font used on the title logo and forum names. I use Arial for some of the smaller text, like the "how may I help you?" on the new meta logo.
The fonts used in the pages body are picked from the following list - depending on your computer (if you don't have the top one available the browser goes down the list until it finds one).
verdana,
geneva,
lucida,
'lucida grande',
arial,
helvetica,
sans-serif