Try http://8aweek.com/ it's designed for doing that (keeping track of wasted time while browsing and blocking after a while) - but I have to admit I disabled it.
I was also a little concerned about the privacy implications of sending them everything I do, and if you disable that (which you can), many things don't function as well.
Edit: WAIT!
What?? What happened to 8aweek!? Now it's socialbrowse, which so far as I can tell is designed to do exactly the opposite of what 8aweek was going to do (i.e. save your time - now they want to waste it).
I was also a little concerned about the privacy implications of sending them everything I do, and if you disable that (which you can), many things don't function as well.
Edit: WAIT!
What?? What happened to 8aweek!? Now it's socialbrowse, which so far as I can tell is designed to do exactly the opposite of what 8aweek was going to do (i.e. save your time - now they want to waste it).
Edit2:
8aweek was a ycombinator startup! What happened?