DansGuardian is a filtering proxy for Linux, FreeBSD, OpenBSD and Solaris.
It filters using multiple methods. These methods include URL and domain
filtering, content phrase filtering, PICS filtering, MIME filtering, file
extension filtering, POST filtering.

The content phrase filtering will check for pages that contain profanities
and phrases often associated with pornography and other undesirable content.
The POST filtering allows you to block or limit web upload. The URL and
domain filtering is able to handle huge lists and is significantly faster
than squidGuard.

The filtering has configurable domain, user and ip exception lists.
SSL Tunneling is supported.