Who wants scads of commercial PC games for free? No, I don't mean the kind you download illicitly, silly. I'm talking about older but as well a few newer games, including the occasional award-winner, that publishers have made available over the years and on the house.
Games like Richard Garriot's pre-Ultima RPG Akalabeth (1979). Revolution Software's Beneath a Steel Sky (1994). The original Command & Conquer Red Alert (1996). SSI's Dark Sun: Shattered Lands (1993). David Braben and Ian Bell's Elite (1991). Rockstar's Grand Theft Auto 2 (1999). Sierra's Tribes 2 (2001). Even S2 Games's just-last-January Savage 2 (2008).
No comment on Wikipedia's value as a repository for factual data, but its "List of Commercial Games Released as Freeware" is both convenient and verifiable. It's an alpha-sorted collection of older and some not-so-old titles with links to their Wiki pages, which in turn link to each game's freeware storehouse.
Have a look. There's Abe Lincoln Must Die! (2007), the fourth in the recently released gonzo Sam & Max series -- obviously bait for the other five episodes outstanding, but worth a look just the same.
Remember Bungie's pre-Halo Marathon Trilogy? Yep, all three, available gratis (and the first one's a whole 4.7MB!).
Pick of the litter? Possibly Virgin's SubSpace. It's a simple two-dimensional multiplayer space shooter, easy to learn, challenging to master, and pathologically habit-forming. It was also still being updated, according to Wikipedia, through 2007.
And who could forget Derek Smart's Battlecruiser 3000AD? After the absolutely wretched and bug-riddled 1997 original, the much-revised Millennium version (2003) was surprisingly well-reviewed (by which I mean even the one or two unrepentantly navel-gazing game mags gave it at least one thumb up!).