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 (5 pages)
There is also 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.