http://www.google.com/history
Google History is an opt-in service that allows you to view not just your search history, but your 
entire browsing history. The Google Toolbar phones home for each page load to let Google know which URLs you're browsing.
However, this is just a legitimate frontend for what Google have been doing for ages - when you have PageRank display enabled, their toolbar already phones home for every single page you visit.
Even though it's only a URL that is stored there's a lot you can tell about the user from analysing their history, and some sites (such as competing search engines) will reveal extra detail due to them using variables in the URL (HTTP GET rather than HTTP POST). If you search for "midgets ass fucking" on Yahoo then the URL will clearly show this.
I wonder how many of you guys in adult have PR (or web history) enabled and just surf away, completely unaware that every single site you visit is being recorded by Google... there's a reason I only have the toolbar installed on IE, which I basically use only to check the PR of a few sites.
Don't forget that plain old Google Search already sets a long held cookie which can be used to tie every search you've ever made to a single PC...
Discuss.