From Plaxo you can centralize your contact information for multiple services as Outlook, Outlook Express, Google Calendar, Yahoo! Mail, AOL, Hotmail, Thunderbird and LinkedIn. When a friend of yours changes their contact info, you’ll see the change automatically (in case your friend is a Plaxo user).  Other features included birthday reminders and some more.

There’s a free basic service and some other premium services with some extra features such fixing duplicate contacts, sending unlimited ecards, getting assistance from Plaxo and many more.
It’s really a wonderful tool!. Get connected!.