I pointed to an early version of this in the Free Games of the Week a little while back, but it's worth a bigger mention now that (VVVVVV and Super Hexagon developer) Terry Cavanagh has finished 'Untitled Stealthy Thing' and given it a proper name. You can play Tiny Heist in your browser for free, and you should, because it's a lot of fun.
Imagine Rogue crossed with Invisible Inc. and you're somewhere in Tiny Heist's ballpark, as it's an ascii-style, turn-based, procedurally generated stealth game where you play as the @ symbol (this time with a pair of arms and legs). Find the key, snaffle as many gems and coins as you're able, and hot-foot it to the exit of each floor, while attempting to avoid (or KO) the various guards, alarms and guard dogs that have assembled to keep ne'er-do-wells like you out.
Tiny Heist is quite tough, by the way—if you take too long in your deliberations, extra guards will enter the room, while KO'd enemies will eventually wake up. Can you make it all the way to the game's 15th floor? No. You probably can't. But give it your best anyway.