Yes, several:

http://www.collegeopentextbooks.org/

http://oerconsortium.org/discipline-specific/

http://open.umn.edu/opentextbooks/

http://opensuny.org/omp/index.php/SUNYOpenTextbooks

http://www.openaccesstextbooks.org/

http://www.opensourcetext.org/

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

http://creativecommons.org/tag/open-textbooks