Some nice suggestions there, I'm going to post some observations on each one to see if they're optimal for me (please don't feel I'm moving the goalposts, if anything it helps refine a bit the guidelines for the games. I'll probably still include it for reference even if they're not our cup of tea):
-Urban Terror: I remember playing it a bit back in the early non-standalone days. A bit like CS in that it's team based, not sure if there's respawns or you stay dead until a "round" ends or something. Advantages vs Counter Strike? Not sure if Q3 engine is heavier or lighter than HL though (which was based on a heavily modified Q1 engine IIRC), I'd have to test it on these machines.
In any case, you've reminded me that there's a lot of mods out there for Quake and HL.
-Battle for Wesnoth: that's turn based, I think? Not a RTS. I'll add another category.
-Terraria: um... yeah, that needs Steam, also I'd have to make each of my coworkers sign up and buy their own copy (probably won't want to do that in our work comps anyway)... it's also a bit too sandboxy.
-AVWW: I'm gonna look it up.
-Serious Sam: well, that's definitely a mindless shootfest if I've ever played one
adding for completeness sake (also the 3D isn't too heavy IIRC).
-Ace of Spades: isn't that a bit too "alpha"? Haven't kept up with the progress of the game.
-Earth Series: hhmmm... I don't remember every playing those, could be interesting to review.
-Metal Fatigue: already included. Also it's awesome
katana-wielding giant robot RTS for teh win.
-Spring: need to check it out, probably should include it along with TA.
-Savage, Tremulous: not familiar with those, gonna check.