Well, this is pretty stupid, but I thought it worth a quick mention as it just came in handy a minute ago.
If you're as anal about your window sizes and positions as I am (it's okay, we all have our things), and if you happen to be changing either the size or the position of a window in the Finder, and if you, for some reason, decide to change your mind and you don't want to resize or reposition that Finder window, just press the esc key, and your window will pop back to its last state.
Hey look at that: a whole post in three sentences.
Followup:
This works as described in Panther, but in Tiger it only works for size, not position. Pressing esc while moving a window in Tiger has no effect.