funny I just posted the same question not to long ago and didnt get many suggestions. I had 10 people and we ended up playing BF3 for about 14hours. two games I found that were free were Team Fortress 2 - off of Steam. And Battle Forge - a free RTS game from EA
you can actually find a some good free games on steam for multiplayer - Alien Swarm (shooter) is another one.
** here just found this- free steam games
http://forums.steampowered.com/forums/s ... p?t=851573
For all Steam users:
Alien Swarm: install - storefront
America's Army 3: install - storefront
APB Reloaded [6][7]: install - storefront
ArmA II Free [1]: install - storefront
BattleForge [1][4][6]: install
Between [1][4]: install - storefront
Bloodline Champions [6][7]: install - storefront
Champions Online: Free for All [6]: install - storefront
Codename Gordon: install
COIL [1]: install - storefront
CrimeCraft GangWars [6]: install - storefront
DC Universe Online [6][7]: install - storefront
EverQuest II [6]: install - storefront
Fallen Earth Free2Play [6]: install - storefront
Global Agenda: Free Agent [6]: install - storefront
Iron Grip: Marauders [6]: install - storefront
Magic: The Gathering – Tactics [6]: install - storefront
Microsoft Flight [6][7]: install - storefront
Mightier [1]: install - storefront
Moonbase Alpha: install - storefront
Pandora Saga: Weapons of Balance [6]: install - storefront
Peggle Extreme: install - storefront
PoxNora [6]: install - storefront
Puzzle Pirates [6]: install - storefront
Realm of the Mad God [6]: install - storefront
Rise of Immortals [6]: install - storefront
Sam & Max Episode 104: Abe Lincoln Must Die!: install
Spacewar: install
Spiral Knights [6][7]: install - storefront
Super MNC [6]: install - storefront
Star Trek Online [6]: install - storefront
Stronghold Kingdoms [6]: install - storefront
Team Fortress 2 [6]: install - storefront
The Cat and the Coup: install - storefront
TrackMania Nations Forever [2]: install - storefront
World of Battles: Morningstar [6]: install - storefront