UCB/LBNL/VINT Network Simulator - ns (version 2)

Download it.
Installation tip
On RedHat, only OTcl, TclCL, Nam are needed since other packages come with the OS. However, TCL/TK may also be necessary because the access to the source of them is required for building OTcl and TclCL.
Documentation
Marc Greis's tutorial for basics.
ns Notes and Documentation for profound exploitation.

Emulab at University of Utah

A remotely configurable testbed with over 100 Linux/FreeBSD nodes.