Shareware for Net Nannies

Hi
I am looking for some free or inexpensive software that will allow me to Manage internet access to various clients, either directly or via our server.
Currently we use DSL via a proxy server.
My need is to find something that bars everything on the net, but then allows me to add the sites I wish. (most programs work in reverse and just allow you to bar what you don’t want)
The upshot would be that each individual client would only be able to view maybe 2 or 3 sites.
Many thanks for yourhelp
Richard UK