whitetrash is a great web security companion for squid, it is great in many ways and I thought I'd throw a plug in for it here.
Not only does it only allow people to visit approved sites it also lets the users (or the administrators) add sites to the approved list via a web form that you get automatically redirected to. As of 1.0 it also checks sites against the Google Safebrowsing API so that sites that Google thinks are bad don't accidently get added to the whitelist. It's got fancy authentication options, captcha support and a firefox plugin. No steak knives though.
So why do you need something like this? Well mostly it's to stop unintended web content from being access and/or rendered. For example injected iframes on compromised sites or spyware trying to post your sensitive data to the bad guy. Basically it tries to cut down on the web traffic that happens without a specific user requesting it. All of what you want but nothing else.
Go look at it.