Looking rather like a software version of Yamaha's much-hyped Tenori-on, Weepr is a new cross-platform application that can be downloaded for free.
If you want to find out more about the software, your best bet is to watch the tutorial video that's been posted on the support website. This shows how music can be created on Weepr's matrix-style interface, though it has to be said that the results are decidedly mixed.
Still, Weepr is free - it's being distributed under the Creative Commons license, in fact, so users are allowed to adapt and build on it - so it might be worth devoting an hour of your time to. It's compatible with Windows, OS X and Linux (you also need to have Adobe Air), so no one need miss out.
You can download it now from the dedicated Weepr website.