PageAddict.com - A tool for monitoring your browsing habits and then allowing you to attach tags to certain types of sites such as "games", "work", "fun", and assigning a limit to how much time you can spend on that type of site per day.
Timer - A very simple extension that does nothing but count the time from when you start it until you tell it stop. Good way to at least monitor how much time you've been in front of your web browser.
TimeTracker - An extension that will monitor your browsing history, but monitors actual browsing time, not the time the browser is open. You can also add work sites that will stop the cloc while you are viewing it.
Usage Counter - Appears simple on the surface, and a lot like Timer, but it does keep track of your stats over numerous sessions, you can also set domains for it not to time.