Image Overlays for Google Earth

Using Google Earth, you can use KML files to retrieve raster images and display them as a transparent overlay on the satellite images. The image will be desplayed and also listed under "Places" in Google Earth's left panel. To change the degree of transparency, right-click the respective entry in "Places", and choose "Edit..." from the context menu.

Map of Greenbelt Park, Maryland (240 KB)
© U.S. National Park Service, converted from the PDF map.
Map of Rock Creek Park, District of Columbia (840 KB)
© U.S. National Park Service, converted from the PDF map at 300dpi. Google Map may display this image only at a third of its actual resolution.

Google Earth is also capable of overlaying 3D objects. @Last Software has an editing software and a model of their headquarters.

GPS-Visualizer has map overlays and line/circle tools. An installable plugin (download page) selects and retrieves overlays depending on your current view.

A Google Map Overlay by Bernhard Sterzbach allows you to use the maps from map.google.com, and display them in Google Earth.