Big companies often use an off the shelf content filtering setup whose support contract includes blocked site lists with regular updates. I used to work for a

Big companies often use an off the shelf content filtering setup whose support contract includes blocked site lists with regular updates. I used to work for a bank that used one called SurfControl (who apparently have been acquired by WebSense: http://www.websense.com/site/scwelcome/index.html http://www.websense.com/site/scwelcome/index.html). So every now and then at work I would return to a site I'd been visiting regularly and it would be blocked after a new URL database update had come through. Sometimes it would eventually be unblocked again some time after that.