I have a weakness for grisly first-person shooters, and few have ever compared with Quake. Now, the compassionate humanitarians at Id Software have create a thrilling online deathmatch version of Quake called Quake Live.
Just the thing after a hard day of ratiocination!
Best of all, it's completely free, doesn't require any special graphics cards, and has no gimmicks or spyware or anything, as far as I can tell (and I'm persnickety). You just download a few small programs, create your character, and begin reducing other players to glutinous globules of glowing goo.
My handle is GiantMetalClown, if you want to frag me (if you know what I mean).