Google has supported searching for Creative Commons licensed content through the usage rights portion of the advanced search interface for some time. Last week they took the next logical step by announcing on the Custom Search blog that you can now use the indexed license information to filter results in your own custom search engine.
Custom search engines allow you to create a search for a set of sites and host it on your site. This improvement allows you to further restrict your results to resources marked as under a Creative Commons license. The announcement also enumerates how Google looks for CC licenses, although content creators needn’t worry about that aspect — the HTML generated by the license engine contains all the bits you need; just copy and paste!