Instructables has a new DIY project for creating a step sequencer for music:
One of the keys to making good music is mindless repetition. That is what the simple sequencer is great at. It does the same thing over and over again in an eight note sequence. You can adjust the frequency of the note, the duration of the note and the pause between notes. If you get really good, you can anticipate the next note and change things up on the fly. This little box is sure to provide endless hours of fun.
The project is probably not for the first timer. Details of what you need in order to make this are available here.