Legend of Grimrock because it was on sale from Steam last week. I'd remembering reading something about it, and when I realized it was an "old school dungeon crawl", I was very, very excited.
I'm pleased to say it's awesome. I've played it quite a bit this weekend and truly love it.
You run 4 characters (created by choosing between Warrior, Rogue and Mage, and the races of Human, Minotaur, Insectioid and Reptoid) through Grimrock.
One of the neat bits is that Grimrock is where prisoners are sent after being given a death sentence by the Empire. An airship takes them to the top and if they can survive to the bottom, they've earned their freedom.
I can't recommend the game enough.