Argus is fully open-source, and may be downloaded and used by anyone at no charge.
Find the current stable version here:
argus-3.7.tgz
[14 Feb 2013 - 436K]
The current development code is here:
argus-dev-20121229.tgz
Older versions are available here:
http://www.tcp4me.com/code/argus-archive/