Dungeon of Zolthan
A little like what Metroid would have been on the Atari 2600: colorful, blocky, short and challenging. The one boss battle I've seen is slightly frusterating; as far as I can tell, there's no way to prepare, so you gotta repeat until you get lucky. Fortunately, the save system is generous and the platforming is fair.
Like the original Metroid, I probably won't finish this. I don't feel like the reward for getting better will be worth the effort. The price is right for trying out Dungeon of Zolthan, however.