Check out the Yahoo MapMixer
I just recently came across the new MapMixer service from Yahoo! which lets you upload your own images and overlay them on top of Yahoo maps. The service lets you pick points on your image and the Yahoo! map to allow you to geo-reference the uploaded image. The generated map can be shared with others and/or embedded in websites and blogs. It is a real simple way of sharing map overlays. The opacity of these map overlays can be adjusted on the map which is pretty cool. The MapCruncher software from Microsoft is much more powerful by letting you generate map tiles that can be used as overlays over Microsoft VirtualEarth. The image tiles, which can get pretty huge, need to be hosted someplace to be used in VirtualEarth. The latest version of MapCruncher also supports uploading the image tiles to the highly affordable Amazon S3 service.
Here are some the Yahoo! MapMixer maps that I created playing around with the service.