Exitfire401 - based mod
I say our options are -
A F2P MMO, or possibly one with a trial period
Some kind of MOBA - (Dota2 or LOL)
A cheap FPS (CS:S, CS:GO, BFBC2)
An RTS (Civilisation series, Age of Empires series)
Card games (CAH, Tabletop Simulator)
Indie games (Speed Runners (only 4 player) showdown effect, etc)