Not sure if it's just software; but a circuit-bending + Arduino project that gets songs and data from last.fm etc. would be pretty cool. Other than that, maybe

Not sure if it's just software; but a circuit-bending + Arduino project that gets songs and data from last.fm etc. would be pretty cool. Other than that, maybe a song finder that finds all the popular songs with the same bpm or in the same/similar key for mixing. I know one of the mixing sites already do bpm which is pretty easy to decipher. The tech to decipher keys is also available. Good luck.