Infested Planet

RTS. Kind of reminds me of the ground Comabt from Star Wars Empire at War crossed with Starship Troopers. Command a squad of space marines tasked with battling an overwhelming alien horde. Pretty fun campaign (if a bit of a predictable story), plus an endless mode. Not exceedingly difficult, but definitely challenging enough to make you think strategically and keep you on your strategic toes. Somewhat limited replayability makes the sticker price hard to recommend (unless your bread and butter is RTS), but it regularly goes on sale for less than $5, which it is absolutely worth!