3 activations only.

OpenTyrian and the original (now freeware) Tyrian here

I imagine that most of these have already been posted in threads already, but here is a list of my favourite free games.
Cave Story (download Deluxe or the Japanese version and the English patch)
http://www.miraigamer.net/cavestory/downloads_1.php
Gravity Bone (Scroll down to find it)
http://www.blendogames.com/
Grotto King (Same developer and website as Gravity Bone)
http://www.blendogames.com/
Harmony
http://rabotik.nl/harmony.htm
Action Doom 2
http://www.gamershell.com/download_33252.shtml
Desktop Dungeons
http://www.qcfdesign.com/?cat=20
Bounce!
http://www.indiedb.com/games/bounce
Bloom
http://www.indiedb.com/games/bloom
Mirage
http://www.indiedb.com/games/mirage
Ayla
http://www.indiedb.com/games/ayla

Flight of the Amazon Queen is now available for free @ dotEmu

The Windows version of the first Wing Commander game is now free, available right here. :)

I am a big fan of old Capcom games, so I really want to share my kind of fun:
Devil May Cry modding Community:
http://s1.zetaboards.com/InfernalWorks/index/
Resident Evil modding Community:
http://z6.invisionfree.com/Resident_Evil_4_PC
Get the Hi-Res and Keypad-mod for Resident Evil 4 and this could be the GotYAY port.
Seriously, Capcom should've done the porting themself, not Ubisoft. On the other hand, there wouldn't be the possibility to mod it.

This thread should stay alive.

Well you can 'sticky' it by checking the 'This is my favourite topic' check box in top right corner of the topic.

Subvein: http://www.subvein.net/
It's a free, GTA-like game in a top-down perspective. Thanks Drelmanes for the tip. ;)
-
Love (MMO): http://www.quelsolaar.com/love/
It does have subscription fees, but there will be a free weekend by this weekend. :p

You're welcome.

Ultrastar Deluxe: http://ultrastardx.sourceforge.net/

A karaoke game remarkably similar to Singstar. A few songs included, though if you have the PS2 games you can even import your songs, and use the Singstar USB microphones.

Edit: While I'm at it, here are a couple of other similar console music game ports that have been out for a while:

Frets on Fire: http://fretsonfire.sourceforge.net/

Inspired by Guitar Hero. Fairly large library of free songs, also allows you to import songs from Guitar Hero 1 or 2. Apparently it's also easy to use a PS3 or XBox Guitar Hero controller.

Stepmania: http://www.stepmania.com/

Inspired by Dance Dance Revolution. Brilliant feature set, thousands of available songs.

Les Miserables is now freeware
http://enterthestory.com/Les_Miserables.html

Don't forget all the strategy games on the Spring Engine. (Evolution RTS, Spring: 1944, Kernel Panic, War Evolution, etcetera)

US only for the moment, the european launch has been postponed...