MagicPrefs: don't use a Magic Mouse without it!
Because the Magic Mouse's main selling point is its versatility, MagicPrefs covers a lot of ground in terms of clicks, taps, swipes, pinches and drags. You can assign functions to practically anything, even three and four-finger clicks and taps. On top of assigning standard keyboard commands, you can also make your mouse activate hot corner functions like Exposé, so you never have to drag your cursor to a corner again.
Apple should build something like MagicPrefs into the Finder, because you're really not getting the most out of your Magic Mouse without it.