And let us not forget S.C.O.U.R.G.E.
It's supposed to be fully finished now.

Just noticed the that the original Command and Conquer was released as freeware as well.

On a more obscure note, more people should try out Glitch, a completely psychedelic FPS.

I like Every Extend, , and Petri Puho's [url=http://www.kloonigames.com/blog/games]Kloonigames (especially Choke on my Groundhog, YOU BASTARD ROBOTS, Jimmy’s Lost His Toilet Paper, & Crayon Physics). In a similar vein, there's some good stuff to be found via indie gaming sites like TIG Source and the Experimental Gameplay Project.
If you want a more mainstream title, you can now get Red Alert for free. :)

I played this game maybe a year ago and thought it was great, but I lost it and couldn't remember the name. I just found it again, so here it is:
Illegal Communication - download is near the bottom of the page. This is a strange abstract action game that forces you to learn the rules as you go. There are no instructions - that's the whole point. It's a short (though sometimes difficult) game and a refreshing departure from traditional gaming, so there's no reason not to try it.

Prodigal. A horror themed adventure game.
Eye of the Kraken. Adventure game on a ship.
Out of Order Sci-fi'ish sort of adventure game. Very nicely done and has an awesome ending too.

I loved Betrayal at Krondor and it's sequal. I'm pretty sure both are freeware now. At least, I'm certain the first one is. Sorry, no links for it because I'm having trouble finding any official ones.
Edit:
I did a bit of searching and I don't know if the sequal ever became freeware or not. I can say that Sierra no longer seems to exist and there's nothing on the site www.sierra.com forwards you to.
There's a few sites you can get the original from. After searching, I realized that I never actually played the sequal and from what I hear I didn't miss much.

Anyone touched on The White Chamber? I thought it was disturbing but great for a free adventure game.

I've downloaded it, but have yet to play it.

Just noticed that <span class="bold">Savage</span> and <span class="podkreslenie">Savage2</span>, two excellent RTS/RPG games, are still missing

I invite you to try Blow Everything Up Net 2007. It's a Polish (but English - language) top-down multiplayer shooter game.
Click on "Zobacz filmik z gry" to watch a movie, and "pobierz grę" (on the right) to download it.

Has anyone seen this:
http://forums.tigsource.com/index.php?topic=4017.0
It's called Spelunky and it's Derek Yu's new game. It's like a cross between a roguelike and a platformer. To tell the truth, I absolutely love the concept and I hope development of this takes off. It deserves to be a phenomenon imo. I'm currently addicted. What do you think?

And for your metagaming needs there is GameBiz 2.
Now you can take your shot at running a game development company yourself.

Homeland just became freeware. It is some indie RPG that has been published by Spiderweb Software.
I never played it but I figured I would add it to this thread in case anyone's interested.