France’s National Geographic Institute has just released a 3D version of Géoportail, a 2D web-based map that was first released last year.
It looks to be a pretty good piece of software. The two biggest drawbacks right now are that it’s Windows only and it has no 3D buildings. However, both of those issues should be resolved soon.
Ogle Earth has a very complete write-up about the software, including some comparisons between it and other 3D earth software.