Last week a co-worker mentioned LinkedIn.com to me, referring to it as somewhat like friendster, but more professionally oriented. It sounded intriguing, but I didn't actually sign up until I read Fred Wilson's post on his "A VC" blog that I grokked? how this site could be of more use. Fred finds it useful for reference checks for potential hires, due diligence, etc.
Anyway, I'm linked in and now have 185,500+ people connected to me by 1-4 degrees of separation.