Disclaimer: I have no financial or other interest in LinkedIn.

On the advice of some colleagues, I've started using a service,
http://www.linkedin.com.  It's a little hard to describe, but I've both
gotten in touch with old friends and gotten at least one active job lead.

There's a theory that everyone on earth is some N degrees of separation
from everybody else, N usually about 6. LinkedIn takes that principle to
N=4. You register, and then set up connections with people you know.  In
turn, that links you indirectly to people they know. It's a distributed
trust model, in that if you want to contact a friend-of-a-friend, the
message goes through your friend, who can choose to forward it or not,
with or without a message. They do up to four degrees of separation.

I think of it as a four-dimensional Rolodex. One colleague described it
as a tool for the times when you think you know someone in a company,
but you don't remember who it is. Others have liked it for getting back
in touch with old friends.