"Good" and "free" are often strangers. Theres lots of cheap, darn little (of quality and not warez) in the way of free. A fair few demos with full ****ional;ity, some ad-ware games like Anarchy Online (if you wanted to try an MMO) and a couple other "free" MMO's (a couple Korean ones), and of course things like FS2 and Allegiance (I think this is still on the MS site, and there used to be a SCP style community around the source, but they seemed more interested in god-modding, it may have improved) where both the source and the game files are available.
I think Bethesda released functional version of the earlier Elder Scrolls games, those are always the time. I think you can get them from their site.
Some java stuff like Megamek (a battletech board game simulator,
http://megamek.sourceforge.net/ )
A couple freebies on the GarageGames.com site, varying quality there from the free ones though.
Now, if you had 10-20 bucks to spare, your choices expand exponentially.