virtual worlds

ArcGIS Explorer looks like it would be a very powerfull program for those in the GIS industry but there is no free version and from reading the website, it is currently only availabe to current customers of ESRI products.
Celestia is different in that it allows you to explore worlds other that our own. Like Jupiter for example:



It also shows how things are in real time so you see the current day/night terminator line. You can also track satillites such as the ISS:



You can't zoom very far in before you lose any kind of usable image though. This service is a fun way to learn about the solar system interactively but that seems to be about the only practicle use.