I loved that one as well.

Also, Ground Control. The expansion isn't free (as far as I know), so GOG still isn't ripping you off.
EDIT: Also #2, Mechwarrior 4: Mercenaries.

Tribes 2 anyone? Awesome game.

edit: and also ET (Enemy Territory)

I don't know if this is the official host for Mechwarrior 4 free but:

http://www.mektek.net/projects/mw4/download.html

I know MS was going to make this free and if you have a Z-Axis joystick... well, what are you waiting for?!!!

EDIT: I should say when Firefall comes out, WOOOHOOO! http://www.firefallthegame.com/home

Yes, this is Youngblood of Tribes fame working on this, the team is awesome, the game is awesome, and it's F2P (only cosmetic items and loot boosts in the shop).

I have a LOT of favourite Freeware games. A few have been homebrewed and some are ex-commercial games. I'll keep it to ones I believe are fairly obscure, but just a quick mention must go to the fantastic 'Spelunky' which having glanced through this post quickly, I can see a few people have already mentioned. Please check out Spelunky. It's Here.

My first choice is quite a bit strange but it holds a special place in my heart:

Seiklus

It's a free-roaming platform adventure game where you are a nameless character who is dropped into a mysterious world. That's it. That's all the information you are given.
And that's what I love about it! It adds mystery to an otherwise incredibly simple concept and keeps you playing.
In this respect it reminds me of newer games that you have to pay for, like Limbo or Blueberry Garden (which you might check out on Steam if you like Seiklus?)


Next is a set of point-and-click adventure games that are completely free, written by an internet critic who routinely talks about how rubbish point-and-click adventures are and also - as a series - quite high up in my top 10 adventure games list (in my head...)

The Chzo Mythos Quadrilogy

For those not in the know, that guy who talks very quickly about how bad games are over pictures of stickmen on a yellow background on the internet review show 'Zero Punctuation', Ben 'Yahtzee' Crowshaw used to make adventure games.
Four of the games on the page I have linked to, make up the Chzo Mythos.
The games are 5 Days a Stranger, 7 Days A Skeptic, Trilby's Notes and 6 Days a Sacrifice. Since Yahtzee hit the big time, he has released all of the Special Edition versions for free too, which is nice!
If you like Japanese horror tropes, survival horror games or grown-up point-and-click adventures, you will love them.
I was surprised of how on edge the games made me despite only having 16-bit style graphics. I often jumped at scenes, especially in the third game Trilby's Noithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsith urtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsi thurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurtsithurt sithurtsithurtsithurtsithurtsithurts


Sorry about that, I'm not sure what came over me there.
Anyway the next game is an ex-commercial release, but I'd never heard of it before. It's:

Vantage Master Online

A turn based strategy game with a 'collect'em'all' aspect that keeps you coming back for more. It's very japanese and the story segments of the game are sadly untranslated but it doesn't really effect the overall enjoyment of the game.
I first learned of the game on a site called Gamehippo.com many many years ago. RIP Gamehippo.

Lastly I want to tell you about my favourite Shareware turned Freeware game ever:

Traffic Department 2192

Made in 1994 by Safari Software, TD2192 tells an epic story... through the medium of words!
Yes, sadly the gameplay aspects of this game are a little bit off, but the story it tells during the cutscenes are magnificent despite only being text accompanied by a portrait of the character saying the lines. I don't want to say anything about the story as I'd like you to experience it for yourselves.

Hope you enjoyed this text-wall.

Linley's Dungeon Crawl Stone Soup:

http://crawl.develz.org/wordpress/

Do you know if a 64bit compatible version exists? It seems interesting, but my Win7x64 refuses to run it, stating that it is not 64bit compatible. I can only assume it's a 16bit application.

Edit: Never mind. A bit of googling told me it's actually made for DOS, a fact which is apparently not stated anywhere on its own website. I might fiddle around with DOSBox to try and get it to work.

Hidden & Dangerous

Warzone 2100

It's hard to say what my favorite one is, but this one is really good. Lode Runner Online.

http://www.daggert.net/Folio/Programming/Presage/LodeRunner/Loderunner1.htm

Thats easy, Dungeons and Dragons Online

I would have to say... Beneath a Steel Sky. Definitely.