Thunderbird 3.1 only: Contacts is Mozilla's nifty Labs project that compiles and manages all your web address books in Firefox, and now it's made the logical leap into Mozilla's email client, Thunderbird. It's a really convenient syncing tool for your desktop email.
Just like its Firefox counterpart, Contacts gives you a place to merge your local Thunderbird address book with your contacts already compiled in Google or Yahoo mail, but also Twitter, Facebook, LinkedIn, Plaxo and your system's native address book. It keeps them all in sync after the initial setup, and it's pretty smart about combining contacts with their different profiles across social networks.
Contacts is a free add-on, requires Thunderbird 3.1 to be installed.