Japanese battery manufacturer Mugen Power has made an ingenious new battery extender for the Nintendo 3DS, extending the battery's life 4.4 times over while retaining the original aesthetics of the system. No gaudy grips or anything, just some extra bulk added onto the system. It's not the most organic design, but it's effective at providing more power to the system, and it's one of the few accessories like this that manages to flush with the original device well.
Unfortunately, it's got a 9,800 Yen ($126) price tag along with it, and I haven't seen any import dealers around that are selling it.
I hope that some dealers pick it up; it looks like an excellent add-on for any 3DS user.