http://www.roguetemple.com/z/hyper/Hyperrogue -
You are a lone adventurer in a strange world, where geometry does not work in the expected way. Gather as much treasure as you can before the nasty monsters get you. Explore several different worlds, each with its own unique treasures, enemies, and terrain obstacles. Your quest is to find the legendary treasure, the Orbs of Yendor. Collect one of them to win! Or just ignore your quest and collect smaller treasures.