Recently on a mailing list, somone asked about setting up web filtering for children.
While nothing beats supervision, som fairly effective and simple webfiltering can be setup on a linux based pc by combining squid with the jesred redirector.
Basically, create the jesred files as follows in /etc/squid and add the lines from the bottom file into your squid.conf. This plus std tranparent proxying does quite a good job. Yes, I know theres very easy ways around it, but by the time the kids figure that out, you're not going to stop them anymore ;-)