The Official Google Blog on a new feature called Account Activity:
Every day we aim to make technology so simple and intuitive that you stop thinking about it—we want Google to work so well, it just blends into your life. But sometimes it’s helpful to step back and take stock of what you’re doing online.
Account Activity let's you get a glimpse of what Google knows about you.
It would be interesting to get detailed statistics on your personal online activities. After all, Stephen Wolfram, the chief designer of Wolfram Alpha, wrote about his analysis of his own personal data recently and the insights you can get with that sort of information are priceless.
Google continues to explain Account Activity:
For example, my most recent Account Activity report told me that I sent 5 percent more email than the previous month and received 3 percent more. An Italian hotel was my top Gmail contact for the month. I conducted 12 percent more Google searches than in the previous month, and my top queries reflected the vacation I was planning: [rome] and [hotel].
You have to turn on web history and pretty much allow Google to track you all over the web. Thanks, but no thanks.