Packet Tracer is a Router and Switch Simulator. It is
created by Cisco. You can do lot of network practicals using this
Packet Tracer is good for students doing Cisco Certified
Network Associate (CCNA) certification.
Cisco Packet Tracer is currently at version 5.2 This software is free
for only the Cisco Netacad students, faculty, and alumni. If you want
to register for Netacad, then
CLICK HERE. I have given some of the 4shared
links where this software is archived.