Router Monitor Soft/Hardware ?

If one has a number of computers linked by the ethernet that are connected to a router which is connected to a cable modem wherein all users share one internet connection.
Is there some way of monitoring each of the connected users to determine which users are taking an unfair advantage.
This delimna arises by virtue of the unbelievably fast cable connection, it seems like at least 100 times faster then ADSL , click the link and its on the screen within a blink, therefore downloading say a 4 Megabyte file from a direct source takes about 6 - 8 secs and if one is the only one downloading, it only takes about 4 seconds, therefore held in the excitement of this phenomena we all have to learn some self discipline when downloading.

I speculate that there may be either a software and or hardware monitoring system either for the master or the slaves or all ?

The master machine uses Jaguar but the slaves comprise macs using 9 and 8/ and there a some PC's using various versions of windows.