I prefer to use the combo of tinyproxy/dansguardian/srg

Tinyproxy is the proxy server
Dansguardian filters internet content
srg produces web based reports

It gives a really nice/basic web based text only report that will show 
you all of the sites that your users have surfed, and will arrange the 
users by total bandwidth/ip address. Dansguardian has the added benefit 
of running everything through Clamav to offer some virus protection.

All three are Debian/Ubuntu packages.

Set Tinyproxy to only accept connections from localhost
Point Dansguardian to the localhost Tinyproxy port
Set dansguardian to use a Squid format log file
Point srg to the /var/log/dansguardian/
Block all internet access but Proxy server with M0n0wall
Either set web browsers to use proxy, or set up automatic browser 
configuration

Andy