Marble is indeed interesting, but in my experience it has been very unstable; last time I experimented across Mac/Windows/Linux, on each platform half of the basemaps would not work, and the set of non-functioning maps varied between OSs.
I have not looked at it recently, but the USA's NASA had an open source 3D globe program of their own called
World Wind; that could potentially be worth investigation.
For everyday users however,
GNOME Maps is probably a better solution. While it is 2D only, it does have substantially better satellite imagery than Marble or World Wind due to a
partnership with Mapbox.