You won't ever actually need this skill, hopefully, but learning how to throw a knife and hit your mark definitely leaves you with a feeling of action hero accomplishment. Here's how to do it the right way.
Wired's How-To Wiki states that the key to a good throw, aside from selecting a well-balanced knife, is the grip:
From 12 feet, you'll throw a full-spin rotation. Grab the knife by the handle, as you would a hammer, but keep your thumb along the top edge for stability and to prevent over-rotation.
Unlike say, learning to fold a paper star, this isn't the kind of thing you should just leap into, of course. In addition to their tips Wired has several in-depth resources to share like KnifeThrowing.info.
Even if you're not planning on taking up the art of knife throwing any time soon, you can still enjoy the following video. This clip is pulled from the show Time Warp—think Mythbusters meets stop-motion videography—and shows off some pretty slick weapons throwing in slow motion. Jump to 1:50 to see them in action.
Have a cool skill or trick you're dying to share? Throw a link in the comments below to show it off.
How-To: Throw a Knife [Wired]