Quake 3 Arena even if it is old, it has a nice and fast gameplay.
If you want something free try Return to Castle Wolfenstein: Enemy Territory. It's a team-based FPS. There are different classes (soldier, medic, engineer...). You can choose the one that fits you.
Have fun!
Thx for your answers but I need a single player, I have already a nice Multiplayer game (half-life 2 deathmatch)
What about Call of Duty 4?
For only multiplayer game I recommend America'a Army. That's game have good graphic and it's very realistic.
If you need play offline I recommend Call of Duty 4, but I heard that game have good multiplayer mode. Call of Duty 4 in full graphics options looks very, very good. Builds, vehicles, people looks realistic.