I bet you have felt this way some other time. I have felt this way quite often, last year, and decided to stop it cold. Installed a Firefox add-on known as LeechBlock. It will block browser access to Internet within the time span you select, or for the amount of time you want, and you can set easily exceptions for sites you use in your work.
It has helped quite effectively, in fact, when it was "allowed net time" I did not even remember what I used to waste time browsing about. Give it a try, it can greatly improve your productivity while at work, or even more if you work from home.
Below I show some screenshots of how to set up a weekday block in which I can only browse for 7 minutes in each hour (will probably reduce this to 5 minutes soon)
In the What to block tab, start by putting *. This will block all sites. Then start putting sites you wan't to be able to visit by putting +sitename. I need to visit ams.org, thus I enable visiting it, among others.
Then in the When to block tab, select the desired days, and write a time span like in my example: 09:30-13:00,15:30-19:00 and select 7 minutes every hour if you want to follow my settings exactly. To be able to block only for 53 minutes each hour within this time limits, select within time periods and after time limit in the When to block dropdown.
The last tab has some miscellaneous settings. Just be sure you select Count time spent only when browser tab is active. Very important if you use Gmail in your browser as mail system.
Enjoy your newfound work hours. What other similar tools are you using to increase your productivity, or at least, reduce your 'procrastinability'?
Love thy tools to increase your productivity
Procrastination: Causes and cures
What the 'broken windows theory' has to say about productivity
Time management systems
Love thy tools to maximize your productivity