simple-ology browser bodyguard
www.simpleology.com has a web site blocking tool for internet explorer. The tool is free if you sign up for the free course. The catch is that they try to sell you the second and third course and also send you a daily email. If you can live with that stuff the tool is awesome. I blocked Amazon, Yahoo, Wikipedia, and a couple of other sites that I tend to waste lots of time on.
The course is not bad either, although I didn't find anything new in it. It's basically a tool to help you review your top three goals and then set a plan for the day online. But, planning is not an issue for me - it's following through that I have a problem with! But there may be some value in the course for others. Check it out - on a designated break of course!