Network Monitoring Tools

I started my own private network with a couple of roommates that I recently moved in with (The internet bill is under my name and they give me money to help with the bill). I was wondering if there is any network monitoring precautions (like a program that blocks and or notifies me if a file sharing program is running on someone's computer) that I could take to ensure that there is not any illegal file sharing going on under my internet connection.

I am currently using a D-Link DIR-615 router and a HP G60 laptop running ubuntu 9.04 if that helps

Thank you for your time