On my PC, I use a program called NoteWorthy Composer to put notes on staves. Apart from having a nice straightforward interface, it has the great merit of importing and exporting MIDI files. So, once I've found a tune in abc format, I can convert it to MIDI (using the folkinfo abc converter), then use NoteWorthy to transpose or otherwise mess about with it.

However, my main computer is now a Mac, and NoteWorthy is PC-only. Has anyone got any experience of writing/editing/messing about with music on a Mac, and if so what have they used?