Hello,

The most advanced technology for this is with using Netflow protocol (usually v5). Netflow allows collecting very detailed statistics on network traffic.

If your router doesn't support Netflow protocol you could install a Netflow probe there. It will intercept all network packets, analyze their headers and generate a stream of Netflow packets. This stream can be collected and analyzed by any open source (or/and freeware) Netflow tools.
An example of open source Netflow probe is ndsad. Look for it here:
NDSAD project on Sourceforge
NDSAD NetFlow Probe description and manuals

You can go further. You can rate the bandwidth usage in your network, even in monetary terms. For this you require a traffic accounting system (aka 'billing automation system' or just 'billing system'). One of those, Netflow compatible, is NetUP UTM5 (ISP Billing Software "UTM5").