When you say open source I'm under the impression you're using a Unix variant. If this is the case, you could install Squid as a proxy server and monitor those logs. You could even assign usernames and passwords so they'll have to log into the server to go online. This method is better for accountability in the sense that they'd have to provide authentication credentials to get out to the Internet. If you see something strange it becomes more difficult for someone to say "Someone else was at my computer". Search for Squid and Dansguardian they both work wonders in this scenario
perl -e 'print $i=pack(c5,(40*2),sqrt(7600),(unpack(c,Q)-3+1+3+3-7),oct(104),10,oct(101));'