“Visual Search” is an interesting new technology introduced on the Autodesk Labs web. You just “doodle” a part or block you search for and the search engine finds similar parts. You can either sketch a simple geometry (shape) online, upload a 2D or 3D sample file (DWG, DXF, STL, IPT) or an existing raster image. You can then download the found blocks related to your graphical input.
I was not able to use the doodle tool (it has probably drawn in white color) but the whole concept sounds promising. The important question is about the scope of the final library you will search in.