PageAddict is a Firefox extension, wrote by two neuroscientists, that monitors your Internet browsing and displays a summary of the time you've spend time on each web site for the day and shows graph of your web surfing habits history. You can be surprised about results.
PageAddict add-on is actually Greasemonkey script and it stores all the data locally. Here is what site says about privacy:
If you want to focus your web browsing you can add categories for sites and restrict how much time you spend in each category in a day.
More Firefox add-ons.