I recently picked up Insurgency on Steam when it was discounted to $2. Fairly hardcore military FPS in a smilar vein to CoD, BF, Arma, etc. Still has a active community with upwards of a thousand players online at a time. Played the hell out of it for a month, now I'm onto the Witcher 3 (but nobody ever said that was underrated.. in fact, it's turning out to be one of the best games I've ever played in my 25 year gaming history!)