Synergy rocks my world! For anyone who routinely brings a laptop to the office or has multiple computers being used on one desk, sharing one keyboard and mouse amongst them without a KVM switch is awesome! Did I mention this works cross-platform??
Move the mouse off the left side of my main monitor (as though it had a secondary display) and the cursor moves onto my Powerbook screen and controls the cursor there. Whichever screen the mouse is on is also where the keyboard is active. Clipboard can copy and paste (text only, currently) between machines, and it even converts newline characters on the fly between OSs. You can configure keybindings as well, especially for dealing with the pesky way windows keyboards use the ctrl key instead of putting shortcuts under the thumb where they belong. ;)
Here's an example of Synergy in action:
Click to view
Now to move the Vista and Ubuntu boxes over onto this desk... W00t.
Obviously, adding drag and drop across machines would be a huge undertaking to add as a feature. But how cool would that be?