Nintendo's Wiimote controller has been given no shortage of attachments and shells to turn it from a wand into a sword, gun or steering wheel, but if you take your first-person shooters seriously then Penguin United have an alternative. Their CrossFire Pistol eschews janky shells and instead integrates the Wiimote into a new, gun-shaped form factor.
Just about all of the regular functionality from the Wiimote is retained, so you get a D-pad and 1/2 buttons on the back, an A button on the side and a charging port/Nunchuck port underneath. The B button is the trigger, there's a speaker and vibration force-feedback, and of course there's motion-sensitivity integrated; the only drawback is that it's not compatible with the MotionPlus add-on.
Nintendo Life reckon that, at $39.99 - roughly what you'd pay for a genuine Nintendo Wiimote - it's only really for the hardcore shooter fans, and that most other gamers would be satisfied with a far cheaper shell. Still, it's nicely balanced and does what it's supposed to. You can pick one up now from Amazon.