Safari allows to go back to a previous website using the two-finger-swipe gesture. All of the standard multitouch gestures are very nicely documented in the More Gestures section of the Trackpad system preference:
I was looking for a similar gesture in Finder. During my research I stumbled upon a MacWorld article to discover:

• Use ⌥ + three-finger-swipe to go back/forward in finder.

Question:

This feature is not documented anywhere I looked.

• Are there any other gestures implemented in Lion and common applications (e.g. iLife) besides those in the System Preferences?

• Are these "hidden" gestures documented by Apple in a help file or knowledge base so I can look for them as I wish to learn more?

Let's collectively maintain an answer listing of which apps have specific functions controllable by gestures or gestures and keyboard keys.