TimeTracker is a Firefox extension that can help you to be a bit more productive.
How does it work? TimeTracker displays a tiny clock on the browser's bottom right corner with which it calculates the amount of time you spend browsing the web. This information can be useful to check how much time you really spend on the web or to cut down your procrastination time. What's more, if you hover your mouse over the clock you'll see extended data such as your total browsing time since the extension was installed or since the last time it was reset.
TimeTracker can be configured to ignore certain websites, so for example it doesn't count your browsing the company intranet as a waste of time. Unfortunately time is tracked on a general basis and not according to each specific site. Another feature I missed was the possibility of creating and exporting some more advanced statistics, but according to the author's webpage, both specific time tracking and advanced data are included in planned features for future versions.
TimeTracker helps you control the amount of time you spend browsing the web. Future versions will hopefully include extended functionality.