You might want to check out this video primer first, just to get the idea, or you might just want to dive in and zoom, zoom, zoom into the Tokyo Tower Gigapixel Panorama
Planet Sydney. Planet Joshua Tree. Planet Kyoto. Sadly, you can't visit any of these worlds, but you can make your own.
Science Daily reports that researchers at Carnegie Mellon University, in collaboration with scientists at NASA's Ames Research Center, have built a low-cost robotic device that enables any digital camera to produce breathtaking panormamic images called Gigapans.
Field of View's SPi-V engine (pronounced "spiffy") lets you take panoramic images from places like flickr, and turn them into interactive, full-screen environments. (note: shockwave required)
This utterly stunning panorama of Paris by night (WARNING! 15000x520 image, 1.8mb) is almost too good to be true. You can see so many landmarks it's ridiculous - this version has them labelled for your convenience. I traced it back to Arnaud Friche's gallery of panoramic photographs of Paris, churches and cathedrals, and other cities. There are so many beautiful hi-res photographs here that I won't waste any more of your time talking about them.