“Composing” is perhaps overblown, given that I can’t read music. I’m looking for something that can remember and play back little tunes I sometimes get in my head.
If I had a piano with a memory, or a guitar with a memory, that would be perfect.
There are several flash keyboards online that you can use to play one note at a time with a mouse click. There is an interactive guitar chord thingy that lets you save chords in progression and play them back. You can use those to pick out melody and harmony without having to read music. There is a chrome app called Aviary Music Creator that is fun to play with. You can use the piano sounds mode to create melodies with rhythms.