Argus is fully open-source, and may be downloaded and used by anyone at no charge. Enterprise service and support options are also available.
Find the current stable version here:
argus-3.3.tgz
[23 Mar 2004 - 268K]
The current development code is here:
argus-3.4_RC-20050419.tgz
Older versions are available here:
http://www.tcp4me.com/code/argus-archive/