I was recently introduced to LinkedIn.com via an invitation from a friend, and instantly became hooked on the concept of the site and the potential it offers. Basically linked in is a social network site (shudder...), but unlike MySpace et cetera it is not designed for teenagers, but instead aims to provide a networking portal for professionals trying to make new contacts and maintain old ones in order to help their career prospects.
Recently LinkedIn reached 8 million members and seems to be growing fast:
The LinkedIn Generation : http://blogs.business2.com/business2blog/2006/12/the_linkedin_ge.html
I for one enjoy the service offered by this site and wish them every success with it ;-)
Updated 2020 : note that the above post is out-of-date, given this post was originally published in 2006, but is left here for archival purposes. I have unlinked the now-dead external link. As of 2021, LinkedIn now has over 700 members.