Fractal Sequencer is a Windows application that uses fractal images or calculations to generate MIDI note sequences. It is possible to end up with endless variations of themes. You can let it play the General MIDI synth of your sound card or use virtual MIDI cables to connect it to your favorite VSTi and DAW.
Features are added regularly, users' feature requests are taken into consideration. The program can be used to create beautiful fractal images as well as melodies for which there are a good number of user-selectable scales if you want to confine the musical notes to a certain scale.
Don't be scared off by the Windows 95 look of the GUI, it's the musical output (and to a degree the images) that make this software what it is - a nice inspirational tool for melodies and riffs you might not have thought of yourself.
But not only that, you can have it do variations of melodies that you did in fact come up with yourself. The user manual is updated regularly, also at the request of users, which is a big plus in my book.
If I have to nitpick then I have to say that I miss the ability to create multiple melodies at once on different MIDI channels. So far it only outputs one MIDI channel, but multiple instances of the program can be run to circumvent this. And a VST version would have been nice, but virtual MIDI cables, as mentioned above, work just fine.
In the end at its price point you can't really go wrong if you are into non-conventional ways or conventional ways of creating MIDI melodies.