Tasty Planet. You play the role of the toilet cleaner, but you're not really a toilet cleaner after all -- you're a blob of gray goo that can eat anything that's smaller than yourself.
As you chomp away, you grow -- and as you grow, you can eat bigger and bigger stuff. The first level pits you against microscopic particles; by the time I stopped playing, I got all the way to eating cats and dogs. I know that sounds disturbing, but it's a really cute game, and there's no gore or anything like that.
Supposedly you keep growing and growing until you're able to eat whole planets (hence the name). The challenge factor comes when you realize you can't touch any critter larger than yourself - you'll get "bitten" and become smaller. In the beginning you're so small, that a single touch can kill you. Later on, you're big enough that touching larger animals doesn't kill you on the spot, but it does reduce your size. Each level is timed, so if you're not large enough by the time your clock runs out, you need to start again. As long as you don't touch the larger animals, you should be fine.
All in all, a fun, addictive little game. It's available for iOS, too.