Web Safety for Squid
If you need a complete web filtering proxy solution for your business please take a look at Web Safety - deep content inspection web filtering plugin for Squid running on Linux / FreeBSD / pfSense and Windows (virtualized).
DNS Safety Filter
For non managed network a filtering DNS forwarder may be a good option. DNS Safety allows you to filter access to domain names by categories, prevent access to specific domains and apply different access policies for different users.