(I should also point out that if your keyboard has seen so much use that its key labels have worn off, a keyboard skin is also a relatively inexpensive way to figure out which key is which.)
But not all keyboard covers are equal. Over the past few months, I've been taking a few for extended spins on several different laptops. It turns out that despite their apparent similarities, keyboard covers actually differ in a number of ways, including appearance, fit, and the feel of the material itself.
(Note that because Apple has recently settled on a standardized keyboard design for all its products, a keyboard cover designed for Apple's current MacBook line should fit the MacBook, MacBook Air, and MacBook Pro, as well as Apple's keypad-less Wired and Wireless Keyboards.)
iSkin ProTouch FX Keyboard Protector
iSkin's US$35 is unique in that the company has treated the keyboard skin with an anti-bacterial agent; iSkin claims this "inhibit[s] the growth of odor and stain causing bacteria." In addition, rather than using a clear design, all ProTouch FX models are made of semi-transparent silicone with solid-black keytops and white key labels; you can choose between clear, blue, or pink for the silicone color. These black-and-white keytops make the keys easy to read, but they have one major drawback: If your Mac notebook has a backlit keyboard, the ProTouch FX blocks this light completely. (Although there's at least a tiny window that allows you to see the capslock-indicator light.)