Poacher - Yahtzee's latest is funny, free, flippin' hard
Started playing this last night, after about 3 hours I'm 30% through the game. The gameplay is a Metroidvania kinda thingy. Or Shadow Complex kinda thingy if that reference tells you more. As Derek Badger, you run around a huge underground labyrinth shooting monsters and unlocking new moves which lets you access new areas and help the good folks of The Spirit Kingdom fight off the invasion of the Dark Ones. Yeah it's not exactly original but the gameplay is tight and hard(not quite SuperMeatBoy hard but still harder than most games) and a lot of the dialogue is genuinely funny.