Most people will have seen Google Earth and many will have seen NASA's WorldWind. What you will find here is not real time but looks better (I hope). I use similar data to these applications but render them out with various bits of software to give animations and stills to illustrate the treks I have been on in many of the worlds mountainous areas.
The generated landscapes are derived from DEM's. These are Distance Elevation Models. Basically this means that for each point on the ground the altitude is given and the landscape can be constructed. The critical factor is the spacing of the points. The whole world is covered by a spacing of 1 Km but the 90 metre spacing has gaps. Even where the files are available, they tend to have holes where there was cloud at the time the measurements were made. High mountainous areas are even worse and all the high peaks seem to be missing. This is partly due to the high slope values that do not reflect the radar back to the receiver. I have patched these holes with the 1Km data but things get smoothed out and there is not the detail. Fortunately this problem has been overcome by Jonathan de Ferranti who has patched all the high mountain areas very accurately. He has made his work freely available at the link below.
All the data used to make these pictures is available on the internet free of charge. Finding DEM's and getting them in the right format can be tricky.
Useful links are listed below.
Vue From e-on software. My preferred rendering application.
World Machine for generating terrains that can then be imported into Vue and Terragen.
Geekatplay Studio for loads of helpfull video tutorials
www.viewfinderpanoramas.org This is the site of Jonathan de Ferranti. Here you can download the patched DEM's of high mountain regions as well as read about how he does it.
www.planetside.co.uk The home of Terragen another very good rendering programme.
Global Mapper I use this for preparing DEM's for rendering, it has many functions and although a bit pricey for a hobbiest I find it worth it.
World Construction Set This is the 3D Nature site and shows what can be done with WCS and its big brother VNS. I no longer use this as development seemed to stop but it is the best for GIS work. Visual Nature Studio looks good but is expensive.