A seamless P-16 curric...er...wikibooks!
If you found yourself needing an old biology textbook and couldn't locate your battered copy from college, you'd have a few options.
You could go to a university bookstore and snag a used copy; you could drop a few dollars on a new one at Amazon.com; or you could track down some old college chums and ask for their copies.
But if Jimmy Wales and his colleagues at the Wikimedia Foundation have anything to say about it, you could have another way to go--the Wikibooks project. It's their attempt to create a comprehensive, kindergarten-to-college curriculum of textbooks that are free and freely distributable, based on an open-source development model.