If you're the type who wants to sharpen knives yourself with water stones, you'll quickly learn that a proper angle is key. Luckily, the beautifully versatile binder clip is perfect for maintaining the right balance while sharpening.
This actually came up in our comments before, but reader Jon tipped us to a video on About.com showcasing how a quarter-inch binder clip gives you just the right angle for your standard chef's knife on a whetstone. Clip the binder to the non-sharp side, and you can use it as your guide while running the blade down your watered stone. The video can't be embedded and usually features a pre-roll ad, but it's a good demonstration to watch before going ahead.
Sharpening Knives With a Whetstone [About.com]