iPhone/iPod touch only: Microsoft has made a surprising leap into the iPhone app realm, releasing a mobile viewer for its super-high-resolution Photosynthservice. Unless you're familiar enough with the previously mentioned Photosynth tool to create your own in-depth, virtual-tour-ish images, you'd likely use Seadragon as a kind of low-lag image viewer, which it seems to work quite well at. Seadragon comes pre-loaded with some pretty eye-widening images from NASA, the Library of Congress, maps, and intriguing shots from Photosynth, but you can also add your own RSS image feeds—such as those from Flickr, and get similar super-zoom features, up to a point. It's also worth noting that while Photosynth previously required a Windows-only plug-in, it's recently opened up a Silverlight-based viewer that should work for Mac clients as well. Seadragon is a free download for iPhones and iPod touches running at least the 2.0 firmware.