Keepmeout is pretty cool service if you want to block any particular website on your computer for sometime. What they say is if you are addicted to visit a site more often and waste your time e.g orkut use this but then they also give you a temporary URL to access it but If i have to access it I will use that url.
Temporarily Block websites
Alternate URL to access blocked sites
Instead of that use it to block some sites so that others cannot use it. Lets say you have couple of people who come to use your computer for 10 minutes and end up using for hours on some site, just block it with Keepmeout.
However remember this site blocks if you visit the site more than once. So make sure you have accessed it once so your friends see it blocked when they try to.