Adult Swim has another hit web game on its hands with Escape from Puppy Death Factory.
It's essentially a tribute to Metroid ... with, y'know, puppies. Just wait until you see the ship -- it's priceless. The twist is that you swap positions with objects shot by your gun. I don't know if it's too early in the morning or what, but the difficulty seemed to escalate rather quickly.
If for some reason you want to make yourself cry, rescue a single puppy and return to your ship. The ensuing cut scene is absolutely heartbreaking.