just paid for full features; i've always loved stuff like this. There's a web app I always liked that was just a grid with one instrument only, just a reverby mallet-sounding instrument (looks like they added drums now) http://tonematrix.audiotool.com/

When you're restricted to just a scale rather than all 12 notes in the full octave available to you, it's just so surprisingly easy to come up with little riffs that sound good.