When it comes to exploring the globe, there's usually only one choice on the table that wins hands down - Google Earth.
However, Google's groundbreaking Earth navigation application isn't the only option out there, as Marble proves. Marble is a Virtual Globe and World Atlas that allows you to learn more about the Earth. It enables you to pan and zoom around the planet, inspect landmarks, places and roads, and is enlivened by lots of useful links to Wikipedia articles.
You do all the standard things like measure distances, lay down place markers and obtain coordinates for a specific location. One nice little feature is that you can also observe the cloud cover in a particular location and even lay 'filters' over the globe. These include a topographic map, satellite view, street map, Earth at night, and temperature maps. Marble has therefore clearly been designed more with educators and classrooms in mind than Google Earth.
The downside of Marble is that it's rather selective in the regions it covers. Western countries are annotated and illustrated in great detail, while the rest of the world - especially the southern hemisphere - is lacking in information.
Marble is an interesting alternative to Google Earth that's completely free to use and ideal for educators.