Cisco IOS

Cisco IOS 12.4 Release Info states, “Currently operating on millions of active systems, ranging from the small home office router to the core systems of the world's largest service provider networks, Cisco IOS Software is the most widely leveraged network infrastructure software in the world.”

See: Cisco IOS Usage info

Note: Cisco IOS is not really very related to Apple iOS, despite those names sounding identical (as most people pronounce both names identically, by just saying the name of each letter regardless of its capitalization). Apple did eventually end up paying Cisco for the privilege to use an operating system named iOS.

Cisco White Paper: Cisco IOS and MX-OS Software Reference Guide

Some emulators may exist: There is an “open source” option named the Dynamips emulator, and front-ends including Dynagen and Graphical Network Simulator (“GNS3”). However, these may simply be emulating the hardware, as solutions involving this software still need the software images which are commercial/retail Cisco intellectual properties. Cisco Packet Tracer is software that may come with the IOS images, but the images are embedded and the software's emulated networks don't interface with outside hardware, so getting the images out of that software is not a straightforward simple task.

Perhaps see also: Netvisualizer vs Packet Tracer vs GNS3 vs Network Sims?

Using equipment by others

Some people rent racks of equipment. Forum thread: comment 17 states, “There are actually free rack rentals at packetlife.net ! Check the schedule.” (A hyperlink has been added to quoted text.)

Cisco may also offer this. Cisco's Virtual Internet Routing Lab (“VIRL”) and Cisco Modeling Lab (“CML”) Update from 2010 identifies CML as being a more commercial version, while VIRL was planned to have a freely available option.