Did you know that for years Google has been keeping a record of every search you do? And did you know they're now associating your search history with your Google login for other services like Gmail, Calendar, and the like? Surprise! It's Search History. And now it's being used to personalize your search results.
I don't like Google aggregating this data about me. It is possible to opt out. You can turn off search history recording in the settings page. You can also edit your history, including removing it entirely.
It's still unclear to me exactly when Google started recording these histories under account names. Six tech savvy friends I asked all found they had some sort of history on Google going back as far as eighteen months. Only half of them remember having turned on some personalization feature that would have resulted in that history being collected. A seventh friend who is scrupulous about cookies and logins had no history. He regrets that his privacy concerns keep him from using Google Reader.
I believe search history defaults on for new accounts, so 99% of Google users will have this feature. Probably 98% of them won't even know that their every search is being recorded, associated with their name.
More info: the search history service, search history help, and an inadequate privacy FAQ.
Update Sunday, Feb 11: please read my followup post.