If you have a spare machine laying about try www.censornet.com
Since I've posted about this before, Its probally easier to quote my past message

The distro is designed to be an all in one proxy/firewall solution, and gives you full control over what users see, You can block sites, file downloads. It'll also pick out sites with words in that you enter and block them automatically via the dansguardain program

You can either add users yourself, Or if your running a Windows NT or active directory domain you can import the users from them.
You can set access times, eg don't allow access during lunch hours, Ban users for X amount of days, limit users to a set list of sites and even limit the bandwidth they get.
When users attempt to access the internet, They will be shown a log in box, from which they must type in there password and user name