I think the only other gadget out there that rivals the Nintendo Wii with the amount of 3rd party accessories has to be the iPad/iPhone. A company out of Australia has developed Pix & Stix –a special set of drum sticks and a guitar pick for more realistic instrument play while using the GarageBand app.  Each set includes two drumsticks and one guitar pick which have electroconductive rubber tips for screen pickup.  The  drumsticks supposedly have a "soft-touch" finish that won't damage the iPad screen…but I've seen some pretty emphatic drummers so I'm not entirely sure I'd trust them with one of these around a glass screen.

Pix & Stix is currently in presale mode and will begin production and fulfilling orders once their funding target is reached (already over half way there).  A set of stix and pix is available through the website for $14.95 AUD, about $16 US.