Steam: Robocraft, You Have To Win The Game, Destination Sol, Unturned, Alien Swarm Non-steam: Dwarf Fortress (If you are willing to spend a weekend learning it), WarSow, Hide, Neptunes Pride 2, Spelunky
http://web.archive.org/web/20150220132941/http://www.pcgamer.com/the-50-best-free-pc-games/#page-1 Although not on Steam, Tons of free (as in no money payments at all) games that are incredibly fun and lesser known.
I just realized that this would have been much faster if you clicked my sig which led to my steam profile xP
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