Fortress Forever is a new spin on the old classic Quake: Team Fortress, it has all the regular classes (Spy, Sniper, Soldier, etc...). 

Play typically consists of capture the flag or push styles, with the maturer players getting dominated by the less mature (Dads getting slain by their children for example).

This game requires the source engine (Half-Life 2 or Counter-Strike: Source), but like Hidden: Source it's a mod that is external to Steam, but you should be able to find it for free here.