If you're looking for mindless, traditional FPS action, try Serious Sam and/or Serious Sam the Second Encounter.
They're cheap for what you get and while there is little to no plot or story, the action is just nuts. I just finished Second Encounter and it was incredible.
It's more of the same if you've played Serious Sam 1. I personally found the first one to be easier and much more plain considering it all takes place in Egypt.
The Second Encounter takes place in 3 different locales but it's more of the same run and gun with a few new enemies and a couple of new weapons.
I liked the sniper rifle and the flame thrower. The rifle was more like the rocket launcher but it was a hit scan weapon (instant hit) like the Q2/Q3A rail gun. And the flame thrower was a great substitute to the Tommy Gun or MiniGun.