Open Source Network Monitoring System

Introduction
Another great product from open source community. If you have read Dynagen and Dynamips then this will enhance your experience in cisco simulation. GNS3 offers complex graphical network simulator and launch simulation on them.
Features currently supported by GNS 3 are:

Designing high quality complex network topologies.
Emulating Cisco routers and PIX firewalls.
Simulating simple Ethernet, ATM and Frame [...]

Introduction
It is not directly related to Network Monitoring software. But we can use this software to have better understanding on our network and it is possible to simulate our current network using this software. Of course Cisco product.
Dynagen actually is a front end of a famous cisco simulator software, dynamips. Dynamips designed in 2005 to [...]