Router e-SIM is a PC and Mac program that provides a limited but useful practice environment. It simulates five series 2500 routers that must be configured using the IP addressing scheme of the standard Cisco five-router lab topology.

Router e-SIM is installed on each curriculum PC in the Chabot College Academy.
You'll find it under START | Programs | Cisco Programs | Router e-SIM

With Cisco's permission, Router e-SIM is available free to currently enrolled Cisco Academy students for use on your home PC or Macintosh.

Version 1.1 has been released, and it now supports the number keypad, as well as the clockrate command!

Download it here:
Router e-SIM (PC)
Router e-SIM (Mac)

More advanced simulators are available commercially, including:
CCNA™ Virtual Lab e-trainer™ from Sybex
CCNASim 3.0 Standard Edition from RouterSim