Touted as the address book for everything, this site allows you to sync your contacts easily and through different platforms. The idea is when someone changes his contact information (for example, his email address), all his friends and those with his contacts are immediately updated. There is no need to send "mass mails" to your contacts again when you change email address! Furthermore, Plaxo suports email, phones and social networking information. They also offer different plans for different user levels, from the free basic to the Plaxo Platinum. I think this is a great idea, as sometimes it is difficult to keep track of whom you have informed and whom you still had to. Just update once and all are synchronized.