I'd never seen a six-string bass before, and at that point ... Maybe it's not what you call a "riff," but for me, it's one of the best ways to start a song, and it's one of my favorite guitar ...

(Video embedded below the break.) A few simple parts later (Arduino, MP3 shield, magnetic door reed switch) and a tiny bit of code to randomize which bass riff plays, and it’s a done deal.