Serious Sam HD : TFE is 80% off at Get Games Go as part of their friday the 13th promo.
- Serious Sam HD: The First Encounter = $2.99 / £2.59 / €2.59
The game uses Steamworks, so you’ll have to activate it on Steam.
It has a 68/100 metascore and there’s a demo on Steam. Underrated in my opnion. Great co-op game.