Soundrop isn't really a game -- it's more of a musical instrument, although you can play with it for quite a while. The app presents a small hole that constantly drops balls out at a certain rate, while you draw lines on the screen with your finger. Where the balls hit the lines, a tone is sounded, and by positioning the lines in the right places, you can make an interesting sort of music. The app itself calls it "musical geometry," and it is surprisingly fun, even if you aren't a musician. With headphones, you can actually hear the whole thing in stereo, which makes for another fun factor of the experience.
The app is a free download, although you only get a demo for free, with the ability to just draw white lines anywhere. Paying US $1.99 opens up the full functionality, and you can change physics, add more spouts, or change the colors of the lines that are drawn, creating different musical noises with each color.
It's a good time -- play with the free version to see if it hits you right, and then if you're interested, you can buy in for the two bucks and have the whole app.