BuddyPop lets you quickly access your Apple Address Book via a customizeable shortcut.
It's a small, light and discreet application, has no Dock icon and it is easily and constantly accessible.
With BuddyPop you can quickly find a contact, and drop them a mail, or copy the address to the clipboard.
Screenshots are available in the documentation page
BuddyPop is shareware and costs €10; I've spent a lot of time
working on it, so it's not expensive...
Comments and suggestions are welcome : BuddyPop@tynsoe.org.