Changing the icon size with multi-touch!
I know a lot of people don't have multi-touch trackpads, but it is a new thing in OS X and I am sure that within months, we'll see more and more multi-touch in Apple products.
So this is a tip, but also a warning because it is more useful to know how to avoid mistakenly using this than it is actually useful. This applies to Finder windows that are in icon mode or to the desktop. The way it works is that using the "Pinch" gesture while in the Finder and the mouse is over the desktop will adjust the size of icons. I actually find this somewhat annoying when it happens and I don't notice it until later. I wish there was a way of locking the size of icons (if anyone knows a way, please post about it in the comments).