NodeBeat is free today – normally I think it is a couple of dollars and worth that price!
NodeBeat is a great way just to make some music with your iOS device using a non-traditional interface – forget keyboards and guitars! This is for annoying everyone.
There have been quite a few apps recently that allow for some fun music making but with easy to use and understand interfaces. These apps are amazing for students (and teachers) to explore and create with. I know that when I ask my students to compose their own songs they are very timid and afraid of making mistakes but with an interface like this that fear disappears. Instead, they are eager to explore.