Cyber Gadget's new Wiimote add-on covers your d-pad with a thumbstick. We doubt this will improve the Wiimote's effectiveness in fighting games, but it might make the controller slightly more comfortable for people who just started gaming in the last two console generations, and for whom the d-pad as primary input is weird and alien.
We remember going to the state fair once and buying an adhesive joystick that went over the NES's d-pad. That wasn't a good idea on our part or the manufacturer's! This thumbstick at least won't leave sticky residue all over your d-pad. If you're interested in the Cyber Gadget Remocon Stick, we found an $8.49 preorder link at YesAsia.