Chrome only: Google Chrome extension RescueTime tracks all the websites you visit, breaks them down into categories, and then tells you exactly how much time you wasted on unproductive websites.
To get started tracking your browsing, all you have to do is install the extension, and it will automatically begin monitoring — click the icon in the toolbar for a popup dialog with a quick view, or click through to view detailed stats about what you've been wasting time on. You can also pause tracking in case you need a little, uhh, break from being tracked, though Incognito mode would be simpler for that purpose.
RescueTime for Chrome is a free download, without a login required — all of the data is connected anonymously and isn't linked to any particular user, but you can still delete your data if you choose. If you aren't a Chrome user, you can check out our five best time-tracking applications, including the desktop version of RescueTime.