There's a great list over at <a href="http://www.liberatedgames.com/gamelisting.php">Liberated Games</a>
Furthermore, some of my favorite free (as in speech) games are <a href="http://www.bzflag.org/">bzflag</a> (Irix, Linux, *BSD, Windows, Mac OS X, and more), <a href="http://www.reptilelabour.com/software/chromium/">Chromium B.S.U.</a> (Windows version available but "unsupported"), <a href="http://trackballs.sourceforge.net/">Trackballs</a> (haven't tried the Windows version, but it's there), <a href="http://www.nongnu.org/crack-attack/">Crack Attack!</a> (It's under developmental transition right now, so I can't find the Windows version on the site...I have it sitting around though, so if you're interested then PM me with your e-mail address), and, lastly, <a href="http://www.frozen-bubble.org/">Frozen Bubble</a>