Those amazing games no one knows about.

[Arjahn]

It probably doesn't really count since it is actually sort of popular, but only 2 of my 159 Steam friends own it so screw it, buy Torchlight 2 when it's on sale on Steam. Seriously, it's amazing. It's pretty much Diablo 3 where they didn't fuck up, and it has more classes, items, abilities, and no DRM or Real-world-money crap. You can also mod it to your liking and have up to 6 for online co-op.
Wait for it to go on sale, usually it's only $5-$10, normally it's $20, and try out the free demo.