From Simeon Leifer:
Directions is a set of plug-ins for the Apple Address Book that generate driving directions.
Simply click on an address and select which driving directions you want. Get directions from the address you clicked on to your home or work, or get directions from your home or work to the address you clicked on.
You can choose to have the directions open in your browser or just copy the URL for the directions to the clipboard. Great for emailing to a friend or colleague.