FontPark is a great big database of free fonts — for personal and/or commercial use — that'll work with Windows, Mac and Linux.
With 70,000 free fonts it can be hard to narrow it down, so FontPark has a bunch of filtering and sorting options, so you can browse only the top fonts, for example. You can also search the site if there's something you're looking for in particular. If your typographic gluttony doesn't cap off at 70,000, you can also find great free fonts at DaFont, The League of Movable Type and Urban Fonts.
Got any places to find great free fonts? Share 'em in the comments!