Network Monitoring Tools?

Is there any software, preferably freeware that does a great job of monitoring a network. I have two computers, one I use and the other is in my roommates room. We share a wired connection, it would be nice to see if he is offline so that way I can shut down my modem at night. The feature I am looking for, is like Norton's Network Map, it allowed us to see who was online and connected to the router and when they shutdown. Something like that nothing complex, easy and simple to use.