Believe it or not, it is google.com. It's their "spider" coming to visit CP. We now allow our pages on the forum to be indexed on google as well as other major search engines. Why? Because this increases CP's exposure, making it more known to people on the internet.
You will see googlebot as well as other bots quite often. You will not be able to PM them, they are not registered users, they are simply robots.
Hope this clears things up.
- Rod