"In a nutshell, Temptation Blocker is a piece of software that lets you choose certain programs you want to block from yourself for a set amount of time. So, if you just can’t stop playing that new PC game you bought, even though you have that deadline you have to meet, just block it for a couple hours using Temptation Blocker and get your work done despite yourself."
Temptation Blocker seems to get good reviews from the people who use it, based on the comments on Howell's blog.
I'm a bit too much of a technophobe to try it myself - but the idea seems great. Please let me know whether you find it helpful if you decide to check it out.