FretPet is a guitar-oriented music sequencer and expert system for MacOS computers. With it you can visualize scales, build chords, see chord fingerings, develop chord progressions and picking patterns, transpose and harmonize chords, and perform live polyrhythmic improvisations. It acts as a MIDI controller and can export both MIDI files and QuickTime™ movie files.
It has a diverse array of tool palettes that open up dozens of aspects of the chords that you build. As you build chords on the guitar neck you can insert them into a chord progression and directly apply picking patterns to them. Everything is interactive, so you can play the composition back while editing other parts of it.
For guitar reference FretPet is an amazing expert system. For one thing it can display all the names for any chord, which makes it really useful for transcription. It can instantly move chords through the scale harmonies and transpose by various intervals, playing a picking pattern in a loop all the while.
The program takes some time to master, but it comes with a really nice manual in HTML. After the basic tutorial you should have the hang of it. And check out the Demo Sequences that come with it too. They're fully editable so go nuts.
If you're interested in music theory and guitar or just weird compositional tools you should check it out at http://www.fretpet.com and let me know what you think.
I know this is a shameless plug. But hey, it's Good Software™.