One of Lion's hallmark features was its introduction of a slew of multitouch gestures. By now, you may well have learned most of them. But Keir Thomas discovered one you probably don't know about, a hidden gesture that lets you quickly return to your most recently used space. If you use the four-finger swipe to move from your main desktop to another, or to a full-screen app or the Dashboard, you can quickly go right back to where you were before by executing a four-finger double-tap.
But if you try that right now, it won't work. First, you need to fire up Terminal and paste in this command:
defaults write com.apple.dock double-tap-jump-back -bool TRUE;killall Dock
The first part of that command enables the double-tap jump-back setting; the second restarts the Dock, which puts that change into effect. To try out your new gesture, swipe to the right with four fingers to reveal the Dashboard, and then double-tap with four fingers to come right back here again.