GNS3 is an excellent tool for graphical network simulator that allows simulation from simple to complex networks.A complementary tool for network
engineers and people wanting to study for certifications
such as Cisco as well as Juniper.
It can also emulate many Cisco IOS router platforms , PIX/ASA Firewalls and Juniper JunOS
The program is an open source, free program and may be used on
multiple operating systems, including Windows, Linux, and MacOS X.
Check GNS3 documentation website for more details.
My machine has Ubuntu 10.10 maverick (32-bit ).
