Both thesoundtestroom and iOS MARS released demos of AUFX: Dub! I'm deeply conflicted about trying to pick which one to feature here; they both have their strengths and weaknesses. The iOS MARS video includes Ryan talking about the app and exploring it. Where as thesoundtestroom has a direct audio feed, so you can really appreciate the audio quality of the app, but no talking or explanation.
Electro Drum Pads 24 is a simple, free (ad supported), Universal finger drumming app. It is so simple I ordinarily wouldn't feature it here, but the demo video is amazing. In 38 seconds he shows off how easy it is to put together a great beat with finger drumming.