Patroneo (which is Esperanto for "Pattern") is an easy to use, pattern based midi sequencer, a program that sends digital "notes" to software instruments such as synthesizers and samplers.
Patroneo is primarily designed for educational purposes, where the main goal is to teach the importance of patterns and repetitions in any kind of music. However, Patroneo is a full sequencer you can use to create real music. The constraints it presents will more likely boost your creativity than suppressing it.
You have tracks with one pattern each. Turn on steps in the pattern to play musical notes. Switch the patterns on and off in a sequence to create a song structure. Connect external synthesizers and samplers to create sounds.