I just stumbled across this the other day: NASA World Wind. Basically it is a computerized 3D model of Earth, with user-selectable imagry. You can choose from true- and false-color satellite images, aerial photos and USGS topo maps. The images are downloaded on the fly as you zoom in, so the program is a bit of a bandwidth hog. The terrain is 3D as well, and it gets finer as you zoom in more. It has a searchable index of placenames, and you can hyperlink locations on the globe. Here is my house. I added a link in GSAK so now I can zoom in to any geocache on the planet and view the terrain! NASA has a few topo screenshots and satellite screenshots of what it can do. You can also go to current world conditions like fires, floods & hurricanes, and also look at historical data, such as before and after data of the Mount St. Helens eruption. There is also a growing list of user-contributed data, such as the Lewis and Clark Trail and the Appalachian Trail.
The scrolling and zooming is smooth as silk as long as you aren't using many other programs on your computer. So far I've visited New Zealand, Australia, Japan, the Himalayas, Paris, Washington State and Maine. It's a huge download (188 MB), but it's not a problem if you have DSL or cable. It's also my favorite price: free! I'd be curious to hear from anyone else that has tried it and hear what they think of it. I'm having a blast scrolling around the planet and checking out places!