Take a look at MapCruncher. A very interesting new map mashup technique from the folks at Microsoft Research. It basically allows you to overlay any digital map image on top of a normal, zoomable Virtual Earth map. As a local Seattleite, nice to see their example of bike trails in this area. More details at their site:
Have you ever looked at satellite photos of a building in Virtual Earth -- and wished you could zoom right in and see its floorplan? Have you ever used VE to plan a trip across town -- and wanted to seamlessly switch from its road maps to maps of bicycle trails, bus routes, or carpool lanes? Have you ever wanted to create and publish your own map mashups -- and wished you had a tool to make it easy to integrate a map you care about into Virtual Earth? Microsoft Research has made this a reality! The Virtual Earth API allows web developers to supplement Virtual Earth's maps with pushpins and lines. MapCruncher brings mashups to a whole new level by allowing developers to import entire maps to supplement the existing road and aerial imagery with detailed, application-specific information. The possibilities are endless: bicycle maps, transit maps, national park maps, university maps, antique city maps, or whatever scale maps you personally find interesting. See our Gallery.