We're looking for a netflow analysis software, free. Any suggestion ?

I guess any linux based machine.
I remember a friend of mine once build a CentOS with
Cacti and Netflow integrated.

nfdump or ntop/nprobe.

The tool we use is not free for most uses, but it is was for what we used it for (its only free for two streams). It is called NetFlow Analyzer. We just set it up and only for our outbound routes for the mikrotik today.

A google search for "free netflow" returned this link, worth checking out:
http://www.networkuptime.com/tools/netflow/

NetFlow Analyzer worked great for our purposes when we had it set up for our cisco router. It needs a decently fast computer.

nfdump or ntop/nprobe.

Do you know any windows software?