Nitpicker is a free IP Flow Accounter.
It started because the commercial accounting software was just not fast,
precise and reliable enough to fit a today's ISP / MSP requirements. 

Here are some of the large benefits of nitpicker: 
 - It is 100% free
 - It is Public Domain.
 - It is designed for speed
 - It is designed to run reliable under djb's daemontools
 - It consumes minimal processor power
 - It also runs with little memory
 
WWW: http://www.nitpicker.de/