Alien Swarm is also Free: http://store.steampowered.com/app/630/

List of FREE games on Steam (Click on the link for direct links to download/install) http://forums.steampowered.com/forums/showthread.php?t=851573

For ATI/AMD (Radeon) graphic card users: install - storefront
- Half-Life 2: Deathmatch [6]
- Half-Life 2: Lost Coast

For Nvidia (GeForce) graphic card users: install - storefront
- Half-Life 2: Deathmatch [6]
- Half-Life 2: Lost Coast
- Portal: First Slice (demo install - storefront)

For all Steam users:
America's Army 3: install - storefront
BattleForge [1][4][7][8]: install - storefront
Between [1][4]: install - storefront
Codename Gordon: install
COIL [1]: install - storefront
Mightier [1]: install - storefront
Peggle Extreme: install - storefront
Sam & Max: Episode 4: install - storefront
Spacewar: install
TrackMania Nations Forever [2]: install - storefront

Free addons and resources for already purchased games:
Half-Life (high definition models): install - uninstall - comparison - screenshot
RACE 07 ... and more....

Games with Steam enabled Achievements: http://v1.steam.hlxgame.cc/stats/

It looks like you can indeed add third party retail games to Steam, however, only certain games are supported. Go to this list to see the complete list:

https://support.steampowered.com/kb_article.php?ref=7480-WUSF-3601

Im not sure ares, but if you find out, let us know.

in Steam there is an option to "Activate Product on Steam"

when I click on it I get a pop up that says "To register your product with Steam, enter the product code distributed with a retail DC/DVD or other Steam product key here. Click next to continue"

does this mean that I can register any retail game (as long as it's also offered on Steam) and it will officially be added to my Steam account? e.g. I bought Rome: Total War retail from a store, will adding the key to Steam register it as a Steam game from now on?

or is this only for official Steam retail games or something?

Anyone have any recommendations for a Guild icon?

I've given (I think) everyone admin access, so if you see missing memebers, please invite them. If you don't have admin access, please let me know and I'll "try" to figure out how to configure it properly.