NeTAMS - Network Traffic Accounting and Monitoring Software. 
It is built for UNIX and works for networks with Cisco routers or PC Unix 
routers (Linux/FreeBSD/Solaris). Several methods of traffic gathering are 
supported (tee/divert/ip_queue/libpcap/netflow v5), database storage 
(BerkleyDB/MySQL/PostgresSQL), aggregation, visualization, notification etc. 
It is possible to block traffic based on quotas, authorization ID, empty 
balance (billing); manage bandwidth, create a flexible accounting and 
filtration policies.

WWW: http://www.netams.com/