I have come across a piece of OSS software for practising skills in keyboard playing. (well any midi instrument I suppose). It is written in java and I have dome tests on Openjava as well as on a Mac. I have done a review here http://sciencensums.co.uk/lifetype/. I suppose I could do a review of lifetype as well, quite nice blogging software. Jaimus allows you to practise skills and get some feedback without having to have a music teacher on your shoulder giving you feedback. I think that especially the Rhythm work is useful for someone wanting to practise skills on their own. You can practise sight reading, Intervals and Rhythms as well as have a go on a number of games that are built in. I have documented some issues about latency which is probably a programming issue but overall, I think this is something that fille a gap in multiplatform OSS.