Whilst I'd love it if there were more music applications for the palm platform than you could poke a stick at, there aren't, but there are lots of supporting applications, or applications that wrap around your music if you like.
What do I mean by this? Well, applications that help you to make music whilst not actually doing any music making. Apps like chord finders or musical databases, or tuners, or spectrum analysers for that matter. There are lots and lots of these kind of applications, and I have reviewed some of them over time.
So, what's your most useful non-music making music application?