If you haven't seen Shodan yet, you're probably not using Twitter as a means to stay current on security issues. Shoot, I don't even follow anyone and I heard about it.
Basically a programmer named John Matherly scanned a huge swath of the Internet for certain TCP ports (80, 21, 23 at least) and published the results in a database with a nice Web front-end. This means you can put your mind in Google hacking mode, find vulnerable platforms, maybe add in some default passwords (or not), and take over someone's system. We're several steps along the Intrusion as a Service (IaaS) path already!
Incidentally, this idea is not new. I know at least one company that sold a service like this in 2004. The difference is that Shodan is free and open to the public.
Shodan is a dream for those wanting to spend Thanksgiving looking for vulnerable boxes, and a nightmare for their owners. I would not be surprised if shodan.surtri.com disappears in the next few days after receiving a call or two from TLAs or LEAs or .mil's. I predict a mad scramble by intruders during the next 24-48 hours as they use Shodan to locate, own, and secure boxes before others do.
Matt Franz asked good questions about this site in his post Where's the Controversy about Shodan? Personally I think Shodan will disappear. Many will argue that publishing information about systems is not a problem. We hear similar arguments from people defending sites that publish torrents. Personally I don't have a problem with Shodan or torrent sites. From a personal responsibility issue it would have been nice to delay notification of Shodan until after Thanksgiving.