This tool creates a record of all the web pages that you visit. This kinda sounds like a dangerous feature for your privacy but Google says its useful to quickly find something that you remember seeing online. Web History lets you look back in time, revisit the sites you've browsed, and search over the full text of pages you've seen.
You can check out Web History here. All you need to make it work is a Google Account and the Google Toolbar with PageRank enabled.