MapCruncher allows you to easily create custom image tiles to overlay in Virtual Earth mashup applications. The custom image tiles can anything you want to overlay on Virtual Earth imagery - for example, your own aerial imagery, parcel boundaries, neighborhood maps, trails, routes, etc. Here's the official description from the Beta download page:
MapCruncher Beta for Microsoft Virtual Earth makes it easy to publish maps overlaid in an application using the Virtual Earth map control. Once you are familiar with the tool, it will take you about ten minutes to generate a new tile layer. Just find 5 to 10 corresponding landmarks on your map and on the Virtual Earth map view, and MapCruncher will register your map to the global coordinate system, warp it to fit a Mercator projection, and generate a set of image tiles that can be seamlessly layered on top of Virtual Earth map control's standard road, hybrid, or aerial imagery. It even makes a sample HTML page for reference.
MapCruncher accepts a variety of vector formats (PDF, WMF, EMF) and raster formats (JPG, PNG, TIFF, GIF, BMP).
Here's an example Virtual Earth Mashup using tile layers generated from MapCruncher from a previous post: