World Wind is a great program for visualizing the earth created by NASA. It is similar in some respects to Google Earth. Overall I'd say Google Earth is easier to use and runs more smoothly, but it is also less powerful.
In general I see Google Earth being more useful as a geography tool. World Wind is a much bigger program (over 150 MB download) and has a lot more features. Both programs rely on online databases for data and therefore require an active connection to the internet.
Both programs seem to reference similar data sets of satellite photos, but World Wind will also display contour maps, MODIS, GLOBE, and NASA SVS (a set of animations showing change over time).
MODIS is a set of satellite images that are updated daily and provide a resolution of 250 m per pixel. The images don't typically show the entire Earth every day, but do show recent images. GLOBE data includes a variety of data types including: temperature, cloud cover, and barometric pressure among others. This data is updated daily and can be displayed for the entire world.
There are also a number of user created add-ons available. World Wind is Open Source and has spawned an active community of users/creators. Many add-ons can be found at the World Wind Wiki.
What's the cost for this great program? Its free.