Yeah there are hundreds of examples of open textbooks for both the K-12 and college levels. It's nice to see a few school districts (and colleges - like Western Governors) officially adopting them and replacing the old books. However, again, thousands of instructors are already doing so individually ('edupunks' :)

Here are some open textbook sites:

http://collegeopentextbooks.org/

http://www.openculture.com/free_textbooks

http://www.ck12.org/flexbook/

http://www.lightandmatter.com/books.html

Related are the many wiki-based books & notes created in or for courses. For example:

http://anatowiki.wetpaint.com/

http://en.wikibooks.org/wiki/FHSST_Physics