Have you ever been surfing the web and happened upon a site with a paragraph or two that seems VERY familiar. Like so familiar, you realize its been copied off of your own damn site without your own damn permission??? Eh hem. Anyway, its no fun, for shizzle. But there is this awesome resource out there called Copyscape that with one little click of your mouse, will search the web to find copies of any of the text that appears on whichever url you choose to enter into the handy search box.
They also have a cute litte "plagiarism warning banner" that you can dowload for free and post on your own site to ward off would be copycats.