America's Army
Ahh the Army's greatest recruitment tool ever! No kidding there are a lot of people playing this shooting games . The graphics are awesome which made it beat out Open-Arena for me. The only thing that kept AA from being the number 1 was the game-style. If you have played Tom Clancy's Rainbow six then you have played AA. At least style wise they are very similar. It is more a combat simulation than a game which bumped it back to number 2 for me. Meaning the deaths are realistic which makes it less fun as a game. Still a keeper if you have another friend that likes playing America's Army as you can squad up and have a good time as a team.
1 Warsow
OK so you are thinking that you never heard of Warsow? That is a good thing as it is a very addictive game. Take Open-Arena but make the models look cartoonish and make it more of a arcade game and you got Warsow. The coding and graphics are tight. The game-play is stellar with several modes of multi-player to choose from. There are tons of people playing it. So why is it better than Open-arena? Well since they cartooned the player models the frame rate was bumped up big time to make the game super smooth even in very high resolutions. It really is a crisp game. You can play it on Linux, mac and windows. There are tons of player made maps available and the server listing is right in the game. I still have this one loaded on my main game machine and my work laptop (that has low end video) and can enjoy it on both.
So there you have it, just a few free games but there are tons more out there. I went through over 20 free games that I found to weed down this list for you. Now it is your turn to get out there and play. Go get your game on!