http://www.workfriendly.net/
So now you can serf all you want and everyone will think you are hard at work on a word document and not posting on a silly forum. ;D
Or there is http://www.ghostzilla.com/ which I think I have posted before..
This site is all about hiding what you're really doing during the workday.
I say if you have to hide it from your boss or coworkers, don't do it. But then, at my work we're treated as intelligent individuals and entrusted to make the proper decisions about how to spend our time. Actual objectionable sites still get blocked, which they would with workfriendly.net anyway.
Even when you are aloud to surf, you sometimes fee bad about it... ;) Also it is sort of a neey hack that does a fairly good job of reformatting pages.
Don't bother trying to access something that has session logins though ;) Or moderately complex Javascript (let alone AJAX :P )
Neat implementation though.