Weepr is a new free music application that is sort of like a software Tenori-On.

Not much information is available yet, and the site is in Italian, but you can download and try Weepr out here. Adobe’s Air is required.

It’s limited in terms of what sounds are available, and timing is a little shaky, but it’s free, and it’s distributed under a Creative Commons license, so you can change it and build on it, if you like.