Personally I am a big fan of NASA World Wind and Google Earth, recently I came across a new application called WW2D that offers similar functionality. WW2D is a cross-platform, free and open-source application that allows you to explore Earth using satellite imagery, topographic maps and images from other data sources (such as: Blue Marble Next Generation, Landsat7, USGS Topo Maps, etc.).
The program can dynamically download data from the Internet, or you can download the information to your local computer for faster offline access. You can also explore other worlds such as the Moon and Mars.