Microsoft Photosynth is a free software application that analyzes a multitude of photos to create a browsable 3D model by identifying overlapping points in the images. The tool allows you to stitch together dozens of photos to allow a place or event to be viewed from multiple angles.
Photosynth works by analyzing each photo for similarities to the others, and uses that data to estimate where a photo was taken. It then re-creates the environment and uses that as a canvas on which to display the photos. The result is a fresh way to organize and share photography — opening up new possibilities for a 180-year-old art form. The potential uses of Photosynth can range from sharing experiences to storytelling and documentation
It takes 75 photos or more to get the optimal experience, but, with big events, one can also rely on crowdsourcing. As an example CNN asked viewers to send in their photos of Barack Obama's swearing in. See the resulting Inagural Photosynth. You can check out other examples at the Photosynth website.
Microsoft has recently announced (May 2009) that they have integrated their PhotoSynth software into Virtual Earth, allowing users to flip between overhead satellite imagery and photographic stitches. You can check that announcement out here