"NetLimiter Monitor will monitor the usage per machine (Windows only).
NetLimiter Monitor is freeware.
However, this software has to be installed on each computer. If you want to collect usage statistics for the entire network on one computer, you will have to setup this computer as a router and route all traffic through this computer (e.g. with Vyatta Community Edition). Or install a router compatible with DD-WRT. Not only can you monitor the traffic for each computer but you can also limit the bandwidth via traffic shaping."