Most of the time, games in steam cost a bit of buck or free to play have in game purchases. however
for free to play on steam, dota 2 (MOBA) , team fortress 2 (FPS), warframe (grindy FPS RPG), Planetside 2 (FPS MMO)
i played a bit of the mighty quest for epic loot, but it has become pay to win soon.
but u should check out humble bundle for bundle sales at time. there are some quality games floating around,
easiest option is to check out what genre you prefer and hope for steam 80% sales on them =] (for those that's not fps)