Yahoo's Creative Commons Search

Looking for previously created content that you can modify, adapt, or build upon? Try the Yahoo! Creative Commons Search, which helps you find content published by authors who want you to share or reuse it, under certain conditions.

http://search.yahoo.com/cc

Google's Advanced Search ( http://www.google.com/advanced_search?hl=en ) has a similar ability, which can be triggered by changing the "Usage Rights" setting.