Google Earth and Moon
Fun, and practical! The latest from Google is Google Moon, which debuted on the 30th anniversary of "one giant step for mankind." Be sure to go to full magnification!
Next, a program you must download from Google (9MB, free): http://desktop.google.com/download/earth/; Google Earth.
Available is a tool where you can overlay images on top of satellite photos. Try importing a plat map image on top of a satellite photo of the area where your ancestors lived, and you will be looking at a photographic map of your ancestors' property! Adjust the size, orientation, and the transparency level of the imported image, so you can match up the roads, rivers, or other landmarks of the satellite image to your transparency. Drag to toggle between the map and the satellite image. This idea is from an Ancestry Quick Tip by Scott Aaron. Thanks, Scott!
For $20, you can get Google Earth Plus, which allows you to integrate GPS information, and print higher-res maps.
On Eastman's I saw a link to a blog about using Google Earth for genealogy; and in particular, "Creating a Family History Tour" with GE. Start here: http://apoetsblues.typepad.com/studentsofdescent/2005/08/intro_to_google.html
What this power is, I cannot say. All I know is that it exists...and it becomes available only when you are in that state of mind in which you know exactly what you want...and are fully determined not to quit until you get it. - Alexander Graham Bell