If you have any on your mind, get em out there and share them all! Flash based games and others such as Battlefield Heroes where you can play for free but pay for extra avatar customisations or such are also welcome for sharing. Not games however where you can pay for extras that give you an advantage over others!
For example, I'll get the ball rolling with a few of my favorite freeware games here:
Cave Story

The absoloute king of indie, one-man team games out there to date is Cave Story by Daisuke "Pixel" Amaya. It's a great blend of Mario like physics along with Metroid and Megaman style shooting games to create the ultimate Freeware title out there scoring multiple endind, perfect difficulty settings and hidden final boss areas. Better yet, it's avalible on PC, Linux, Mac and even CFW PSPs. Great stuff and well worth playing. It'll be coming out on WiiWare shortly enough for a small price but with updated graphics and sidequests. It's compatible with gamepads also.
Links:
Download link and Tribute site (>5MB, Delux Editon ~40MB)
Wikipedia
Screenshots:
Screenshot 1
Screenshot 2
Ikachan

Another one from Daisuke "Pixel" Amaya, Ikachan came a few years before Cave Story. The game is more of an adventure game than anything, where you play as a small squild creature (Ikachan) who tries to get on by with the rest of the secluded depths in which he lives in. Good fun for fans of adventure games and the like, if a little hammy on the hints of what to do next.
Links:
Download site (>5MB)
Wikipedia
Screenshots:
Screenshot 1
Screenshot 2
Yume Nikki

If you've ever heard of games that people either like them or hate them, this is game will hold everyone on either end. If you like it, you will love it, if you don't you'll likely hate it.
Yume Nikki is something of an adventure game. Made by freeware developer, Kikiyama, you explore the main character's nightmares and dreams throughout a psychadelic Nexus system where everything is merely nothingness. I may sound like a tripped out pot head, but the game has to be seen to be believed. Levels are nothing more than purpously made pixelated messes, with strange beings and curious objects everywhere.
There is no plot and no real ending. You simply explore and enjoy the bizzare worlds and mute characters. You've never played anything like it before, trust me.
If you didn't gather from my writing, I adore this game, and it's a sure fire for one of my top ten games of all time because it's genuinely upsetting and disturbing if you let yourself get into it. Well worth a look, but don't expect much, folks.
Links:
Download links (~40MB)
Wikipedia
Screenshots:
Screenshot 1
Screenshot 2
Screenshot 3
Art of Theft

The popular web critic Ben "Yahtzee" Crosshaw is also known for his quizical little freeware games he has also created. Probably the most well known of these, is The Art of Theft, a stealth-adventure game. You play as Trilby, a high class thief who has someone gunning for him. You get to play out these played out acts of theivery with great sense of patience and skill required to pull off a Trilby Rank for each mission. It's a great little game that is somewhat hampered by it's very iffy controls.
Links:
Download and controls (>5MB)
Screenshots:
Screenshot 1