I think my choice of sources for a mashup would be flickr and a Youtube-type video embedder. You could create a map of the library and embed images so that patrons could view photos of an area in the library, and navigate around it in 3D (see NUS link below). This would be particularly helpful in say, the Audio Visual area, where patrons could view information about the feature film collection, the music collection, or the viewing resources etc. NUS University Library have just gone totally 3d with their collection guides (see here) and Google Earth is always good for inspiration (particularly their 'historical maps' idea)...
