Hidden and Dangerous Deluxe was released as freeware to promote the sequel.

Wow, bump for love. Also, here is a mod for Half-Life 2. Dino D Day is what basically happens if Hitler got even more batshit nuts and instead of using the occult used Dinosaurs. Enjoy.

WOW DOWNLOADING NOW!

OpenGL renderer for Unreal Tournament, Deus Ex and Rune.
http://www.cwdohnal.com/utglr/

Hey doomer, are you still updating this list?
Anyway, with the holiday season coming, I wish to let everyone know about Crazy Christmas Balls. It's a really simple-but-fun (and surprisingly addicting) game where you have to toss falling christmas tree ornament balls with a pad into hands awaiting them.
The game is completely in German, but is really easy to get into, so language should not be a problem.

ADOM - Ancient Domains of Mystery.
Technically its Postcardware, but I think it still counts here. ( The game is available for download from the author for free - but he asks that if you enjoy the game, you send him a postcard. )
It is the best Rogue-like I have ever played. Very deep, complex and rewarding, and most of all fun and free!
If you can get past the "bad" graphics, and hard to learn ( but very effective ) controls, you will absolutely love this game, and that is a promise.
http://www.adom.de/adom/download.php3

Ben There, Dan That: http://www.zombie-cow.com/?page_id=17
This is one of the funniest games I've ever played. It's a bit of an homage/parody of old point and click adventure games.
Synaesthete: https://typo3.digipen.edu/index.php?id=986
This game is like the old arcade classic Berzerk had a baby with Guitar Hero and synthesizer.
Cave Story: http://www.miraigamer.net/cavestory/
I know it's been mentioned already, but this is really one of the best indie games out there, especially if you like metroidvania games. Don't miss it.
IJI: http://www.remar.se/daniel/iji.php
Iji is a brilliantly put together metroidvania game as well.
Knytt: http://nifflas.ni2.se/
It's a simple, yet brilliant minimalist take on the platformer.
Sam & Max: Abe Lincoln Must Die: http://www.telltalegames.com/samandmax/lincolnmustdie
Classic point & click adventure comedy.
De Blob: http://www.gamedev.net/community/forums/topic.asp?topic_id=401276
This is now a Wii game, but it's still free on PC. It has some similarity to Katamari Damacy, but you are painting everything with a big ball instead of rolling it all up.
Also, if you want to sift through a lot of free stuff, knock yourself out with these links:
This first one probably has the best, most refined list for finding good free PC games:
365 free games:
http://www.gamesradar.com/f/365-days-of-free-games/a-2008071714293190000
101 Best Free Games of 2003:
http://www.1up.com/do/feature?cId=3109554
102 Best Free Games of 2004:
[url]http://www.1up.com/do/feature?cId=3137482[/url]
2005? Couldn't find anything.
101 Best Free Games of 2006:
[url]http://www.1up.com/do/feature?cId=3148013[/url]
101 Best Free Games of 2007:
[url]http://www.1up.com/do/feature?pager.offset=0&cId=3156339[/url]
101 Best Free Games of 2008:
[url]http://www.1up.com/do/feature?cId=3165201[/url]
101 best free games of 2009:
http://www.1up.com/do/feature?pager.offset=8&cId=3174065

^^This.^^
I absolutely agree! ADOM is a great game.

I love the many ways you can die in that game.
My two top silliest deaths were:
1) Orog broke his leg on a unremarkable dungeon.
- I was trying to kick all walls to find a hidden door, but was not really paying attention. Stepped on a acid trap, had message skip on, kicked air by accident, got hurt from kicking air ( Troll Healers aint that agile.. ) and .. well. Died.
2) Executed myself with a magic missile. I was going to aim that thing down the corridor, but accidentially aimed it at the wall next to me. Was a pretty powerful mid-level wizard, so the magic missile was quite powerful. Put about 20 holes into me...good times!
And to not make this post entirely off topic...
http://www.gearheadrpg.com/
Gearhead, rogue like mecha based game supplemented with a graphical interface, making roguelikes more accessible for those with a ascii phobia.

Curator Defense - In this game, you have to defend a conservative museum from the horde of modern art that comes rolling in waves into the museum. At your disposal you have various turrets to damage the invaders and devices to hinder their progress. If you like quick-action puzzles, you should definitely try it.

I don't know if someone mentioned that, but you can download Sudden Strike 2 completely free from Gametop:
http://www.gametop.com/download-free-games/sudden-strike2
More about this title: http://www.mobygames.com/game/windows/sudden-strike-ii

Quadrax
http://www.cauldron.sk/files/quadrax.zip
Mine Mombers
http://koti.mbnet.fi/pacbros/skitsoproductions/minebombers311_freeware.zip
Biprolex
http://download.kdlab.com/biprolex.exe

I loved that game!