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 a few award-winners 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.
Remember Bungie's pre-Halo Marathon Trilogy? Yep, all three available gratis (and the first one's a whopping 4.7MB!).
Pick of the litter: Probably Virgin's SubSpace. It's a simple two-dimensional multiplayer space shooter, easy to learn, challenging to master, and pathologically addictive. 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. (Hey, even the one or two unrepentantly navel-gazing game mags gave it at least one thumb up.)