I have put together a howto document to describe how the internet access on a linux computer can be filtered, and daily/weekly/monthly email reports can be sent for monitoring purposes. This was meant as a replacement for the program "Safe Eyes" as it would not run on Linux.PDF howto (6 pages)
There are some links to this item on different Ubuntu Wiki pages and even a German translation of the howto on this blog entry
(thanks to David Rekowski).
I recently stumbled over an article on linux.com which also deals with this subject, but without the monitoring part: read it here