I'm a huge fan of Cyborg Gaming's Rat 7 adjustable mouse. One has been my primary desktop rodent for a year and a half now, and it's holding up very well. Recently, Cyborg released an updated model specifically designed for MMO players or anyone with a penchant for extra buttons.
While the Rat 7 has five programmable buttons, the MMO 7 offers 13. It also sports two "ActionLock" switches that set the left and right mouse buttons on repeat. Cyborg's software handles the button mapping, and there's even a World of Warcraft plugin.
Like the original Rat, the MMO 7 has an adjustable body and weight system. Individual body panels can be swapped out for ones with different sizes and textures, and the mouse retains the funky look of the original as a result. The MMO 7 doesn't come cheap, though. Amazon has it for $130, which is a good $50 more than the Rat 7.
Interestingly, the MMO 7 isn't the only orange-themed adjustable mouse making waves recently. Thermaltake is working with BMW DesignworksUSA on a Level 10 M mouse. Little is known about the peripheral's design, but The Verge says the height, size, shape, and angle of the mouse are all adjustable. Otherwise, all we have to go on are a couple of cryptic teaser shots. We should learn more this spring, which is when the Level 10 M is scheduled to hit the market.