MapCruncher is a Microsoft Research project which uses the Virtual Earth API to import entire supplemental maps into Virtual Earth. By supplemental maps, they mean any drawn-to-scale maps you can find, be they bicycle maps, transit maps, national park maps, university maps, or antique city maps, just to show a few examples. But not only that, you can use MapCruncher to add your very own, Do-It-Yourself Aerial Photography to Virtual Earth, too. Their Gallery has even more examples of this.
The process of importing your own maps they call "crunching," hence the name "MapCruncher." To use MapCruncher, you just import your map then find 5-10 corresponding landmarks on your map and VE. MapCruncher will then register your map to global coordinate system and generate a set of image tiles based on your map and mash them up with VE's road and aerial imagery.
MapCruncher support both vector formats (PDF, WMF, EMF) and raster formats (JPG, PNG, TIFF, GIF, BMP). To view a mashup, you'll need IE6, IE7, or Firefox version 1.5 or higher.