Microsoft Photosynth is a cool application / service. What does it do? Microsoft says "Photosynth creates an amazing new experience with nothing more than a bunch of photos. Creating a synth allows you to share the places and things you love using the cinematic quality of a movie, the control of a video game, and the mind-blowing detail of the real world." Though a nice description, it does not really tell you what it does.
Simply put, it takes a number of photos you provide to create as complete of a composite as possible. In good examples making the some of the photos a much more complete picture than any individual photo. Think of making two photos into a panoramic...then multiply it by every direction. Not only does photosynth provide a more complete representation of a subject by expanding the height and width of the subject, but it also provides greater zoom detail when photos overlap at different distances and resolutions.
On the photosynth website you will find a number of examples ranging from national monuments, Obama's ignaguration, etc. The most significant drawback is the resulting synth is publically available hosted (by Microsoft) composite. You can embed the synth into your own website or send links to it to your friends, but it is not a photo / video / other that you can download and distribute on your own. Additionally, though I may have missed it, I did not see a way to restrict access to your synths.
Below I have embedded my synth created from 14 photos taken outside of Heidlberg Germany. I marked on the map where the photos are of on the photo synth site. Photosynth reported that my 14 photos are 100% synthy, meaning that they were all able to be placed together. My German Country Side Synth.
I encourage you to play around with it, try zooming in and out, gliding through the synth, recentering on specific photos as shown by their outlines, etc. It is a neat way to show off a subject.