Software to transcribe music - piano keyboard to music staff

Id like to start to get my Riffs on paper.  Surely there has to be some software which would allow me to play a riff on a graphical piano keyboard and then have it transcribed onto a music staff (and paper).  

So I would like to see a keyboard on a screen (tablet for example), I play it in rhythem and the software represents it as notes on a musiscal staff.

Anyone know of such software.....  If not, perhaps I should develop it.

Gary