I've used Google's Dashboard (http://www.google.com/dashboard/) in the past: I used it to clear my web search history and then to disable it for good in my profile.
Today, I got a link on my iGoogle page to visit Dashboard again, and I did. There was nothing that required my immediate attention, but I was again surprised by the ease of accessing my information stored by google, and to make changes to this information as I see fit.
Yes, google is big, yes they do know a lot. But in all my years of web-wandering I have never seen a tool like this, offered by any other company, for controlling your own private data. Few people ever says this on this forum, but Microsoft, Apple, Facebook and Yahoo also have and continually collect tons of data on our web usage. But no one offers a unified access to your data.
As webmasters, the Dashboard is a useful tool for keeping an overall view of "how" google perceives you. You can spot areas where they know too much or too little.