I only just covered this bad boy in last week's Firefox Friday -- and now there's a Chrome version! RescueTime advertises itself as a 'Productivity Meter' but really it's just a cool way of seeing how you spend your time on the Internet.
All you have to do is install the extension and it'll do the rest. At any time you can click the menu button to see a quick breakdown of how distracted you are as well as how productive you are compared to other RescueTime users. If you visit sites like Facebook or Flickr, your distractedness rises; stick to work-related stuff and you're 'safe'. Talking of 'safe', no login is required to use this extension -- you are anonymous (for all intents and purposes)!
The proof is in the pudding, though: after a few hours or days, check out 'Detailed Stats'. Prepare to be shocked, awed, amazed and disgusted by the damning but beautiful graphs. The amount of data that the RescueTime team and its users have collated, and thus the accuracy of the reports, is quite stunning.
(Incidentally, if you're a manager of some kind, there's a 'pro' version which you could no doubt use to improve the efficiency of your team...)