Unreal Tournament 2004 for the pc is one of the greatest games i have ever played, and by far for the longest time. I can easily play this game 4 hours + a night, even on weekends. the main reason for this is also a big selling point in the game: content.
UT2004 ships with a program called UTEditor, allowing you to design cutsom game content, from Maps to Mods (modified versions of the game, whole or in part). If you play a map, and think you could do a better job, it's as easy as opening it up and tinkering around.
The barebone game itself ships with multiple game modes: Deathmatch, team Deathmatch, Ctf and vehicle ctf, Invasion, and assault. With the ease of writing mods for this game comes the abundance of servers, each with their own style and feel. Custom monsters, new weapons, different maps and sounds, it goes on and on.
Hardcore gamers will rejoice in Deathmatch and Assault modes, each with their own rules and limits. For 20$, this game is a steal and will surely offer any fps enthusiast hundreds ofhours of gameplay, minimum.