The digital globe app Google Earth (download) is getting new imagery from inside buildings. One, at least: a new layer gives viewers gigapixel (ultra-high-resolution) photos of artwork from inside the Prado Museum in Madrid, Spain. The Prado project also includes detailed exterior views of the museum's buildings. See Google's blog post on the new layer.
As cool as it is to be able to view the art, the Prado project presents neither a complete (or even close) catalog of art from the Prado, nor an immersive 3D experience. There are 14 images of masterpieces, and you navigate to them via a pop-up that appears when you click on the exterior of the building in Google Earth.
I found that the high-resolution imagery, both of the exterior of the building and the pictures, was too much for my old ThinkPad T60 to handle. Google Earth stuttered and became unresponsive. More current computers should do better.
Other projects that attempt to bring the inside and outside navigation together include Microsoft's Photosynth, which intelligently joins photographs into navigable 3D online scenes, and Everyscape, which creates interior 3D scenes from photographs for businesses.
Google has also turned Google Earth outward, by adding Sky View to Google Earth.