I've been using Wolfenstein Enemy Territory for years its an excellent FREE game thats about 257 meg in size. You have to install a number of patches when its installed namely 1.01. 1.02 & 2.60.
Its a first person shooter PC Based game where you can play as a medic, soldier, covert ops, enginner & Field Ops.http://en.wikipedia.org/wiki/Wolfenstein:_Enemy_Territory
The link above will take you to the Wiki Site for it and the links to download are at the bottom. To play online some servers will ask you to install punkbuster, and you can download and use either Ventrilo or Teamspeak in order to talk in real time to teammates.
There are several clans out there if you want to get involved and play as part of an established squad. If you want further details just ask. I've been playing for a number of years and find myself coming back again and again to it.