FTL: Faster Than Light - just started playing this again. Possibly as close to being the perfect game as you're going to get, and a full playthrough only takes a couple of hours. One of those games you can play again and again. Only £6.99 on Steam, and an absolute must-buy at that price. They've added a bunch of new content since last I played too, which hasn't hurt the gameplay at all, which is the worry with anything so well balanced. Seriously, just buy the damned thing and thank me later.
Also, give DoomRL a try too. I'm not a fan of these ASCII Roguelikes, no doubt because I'm a graphics whore, but they added Doom graphics and sound effects to this one and it makes a massive difference. Tremendously good fun to play, and it's FREE - http://doom.chaosforge.org/