For my school, I recently installed Dansguardian web filter and Squid web proxy/cache. on the Dell desktop computer shown at bottom in this photo.The server is configured as an Ethernet bridge using Ubuntu’s bridge-utils package. That way, the machine is invisible on the network so I don’t have to reconfigure the clients behind the filter.
John Rucker’s howto install DansGuardian was incredibly useful and my primary source.
(Picture: Test Setup for Web Filtering Installation, Originally uploaded by mgolding)