You can create as many "Groups" as possible for categorizing your web browsing, but the defaults are Procrastination, Communication, and Search. Then you can assign any web site you visit regularly to a category. In time, MeeTimer will give you a very well organized overview of where you've been doing all of your surfing, and if you've categorized items, it should also help give you a quick look at whether the time you've spent has been productive or not.
I've noticed a few buggy glitches with MeeTimer's tracking so far, but it's also still fairly new, and in general they seem to correct themselves eventually. MeeTimer is charityware (proceeds go to a charity, and the author says donations will ensure more features), works wherever Firefox does.
MeeTimer [Mozilla Add-ons]
</img>
More...