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.

The best resource I have regarding gestures in OS X is: http://www.danrodney.com/mac/multitouch.html

Not really a gesture in itself, but I have found that certain gestures (at least on my Magic Trackpad) do not require the requisite amount of touches for the duration of the gesture, just during the initiation, for example starting a 2 finger scroll will happily continue if you then lift one of those fingers off the surface and continue scrollng with 1 finger, giving a more iOS feel to the movement.