I have a pretty limited library of multiplayer games on steam:
CS:S (never play)
TF2 (never play)
GMod
XCOM (never done multiplayer in it before)
Planet Explorers (Early Access, multiplayer may need tuning before it's ready for prime-time)
Also, I only do games available for Linux these days.
Sadly, I wouldn't know where to start; I'm no web developer and don't do server stuff. I am tempted to flesh the game out a bit, though, and add multiple stages. I really am happy with the core mechanics of it.