Flickrlilli is a search engine interface that allows you to search the Flickr photo database by any combination of Creative Commons license types. You can search for images by tag and sort results by date or interestingness.
Results appear as large thumbs on the left with the image descriptions and summaries on the right. A direct download link to the original full sized image is available. Clicking on a thumbnail replaces the image with a larger version.
Update: Flickr now has a Creative Commons-specific search on the advanced search page.