I absolutely despise the idea of this, but it can be done pretty easily and for free using squid as a proxy server.

Here is a link to a paper on how to set this up.

Of course, the DSL will need to come in to the proxy computer and the clients will need to be configured to use that machine as their internet gateway and the browser client will need to be informed that the gateway machine is a proxy, but all of that is pretty easy. You can have an OpenBSD box with two NICs doing that job in an hour or two.

FWIW,

Jim