I've found myself wondering on more than one occasion if an issue I'm having is a result of Network security configuration (Routing Rules, Network ACLs, Security Groups) or some other problem. I've also had the need to demonstrate the security I have in place within an AWS VPC. For these needs and more I wrote a network modeling tool that helps you trace traffic flow from a source to a destination simulating security evaluation at each hop. Hopefully others will find it useful.
http://awstools.jacobmarks.com