Over at Instructables, user Banjomaster shows how to make a fisheye lens for his Nikon D90 for just $16, with the help of one of those wide-angle doorway peephole lenses.
Designed for police to look inside your house (illegally?) without letting you know they're spying, the reverse peephole viewer un-distorts the convex lens that's designed for homeowners to view out, not in. You'll definitely find a use for this, for instance whenever that guy Bob pretends he's not home so he won't… »
Pick up a peephole viewer at Home Depot for $11 and suddenly you have yourself a 160-degree super wide angle lens. This is shown working with one of the Canon Digital Elph cameras, but it will probably work with other pocket cameras, too. You simply hold the peephole up to the lens, then zoom in, take your shot and… »