Cisco Packet Tracer 

Simulate a network with an unlimited number of devices and practice troubleshooting on it. The utility embeds application layers protocols like DNS or HTTP as well as basic routing with EIGRP, OSPF, and RIP.

Packet Tracer 5.1 is the latest version of Cisco's simulation software. The main objective of Packet Tracer is to serve as a support tool for the Cisco Academy. This tool is extremely useful for both students and teachers. Basically, Packet Tracer allows you to build a network with a range of simulated "real-life" equipment (Cisco equipment, of course), so different configuration options can be tested. You get a range of routers, switches, end-client systems and connections to build the simulated network. The operating systems of the routers and even some portions of PC's are also simulated. This way, users can learn to configure routers and see the changes they make on the networks. Teachers can even make an interactive test using the program, which can be graded immediately by the program. It is one of the most complete tools for network learning; however, it can also help simulate and assess equipment options for real networks. It is really a tool to have; however, in order to download it from the original site, you need a Cisco Academy account, but the program has a free license.