I've heard Desktop Dungeons described as a 10-minute "dungeon crawl" or "roguelike," and that's really not a bad summation (though it's definitely closer to the latter). You take your character (picking race and class) and attempt to navigate your way through the dungeon and ulti mately take on the boss character (they're at level 10, you always start at level 1). The only way you'll do this is by carefully using what potions and spells you're able to find or buy and very carefully working your way up to as close to level 10 as possible. It's very challenging, but doable once you get the hang of it (unless the dungeon randomization really stacks the deck against you impossibly, which it can do). As you clear the levels, you'll unlock more areas, challenges and classes to play, so it's fairly addicting. Probably one of the biggest things going for it is its length. If you're looking for some bite-size gaming that actually satisfies your sense of progression, Desktop Dungeons fits the bill.
It comes in Windows and Mac flavors and you can download it for free at DesktopDungeons.net today.