Flickr was once one of the best places on the internet to find new Creative Commons-licensed photography, but since its takeover by Yahoo!, it is harder to search and to collect attribution for Commons-licensed content. (See this BoingBoing post for more info on how Flickr is broken in this respect.) The “Explore–>the Commons” tab seems to only search public domain images, or resources from institutions or organizations, neither of which are usually properly marked with a useful license type. The bookmarklet in this post from Digital Inspiration does a good job of making attribution easier, and Cory at BoingBoing has an update to another bookmarklet as well, although I haven’t gotten it to work yet. Librarian By Day has a helpful Flickr/CC attribution guide. (NB I have not always properly attributed here, but I’m at least beginning to learn how to!)
Wikipedia also maintains a list of public domain map resources. If you need to do a great deal of map data work, another good resource to consider is Natural Earth, whose maps include integrated vector and raster data.