An Acronym for Internetworking OperatingSystem.

Written by Cisco for use in their networking devices, such as routers. Purportedly the most ported OperatingSystem ever, since it gets ported to almost all devices Cisco develop internally. However, it is not ported generally to devices only sold with a Cisco label, but actually developed at subcontractors or acquired companies.

A selection of Cisco devices not running IOS

Any device labeled Linksys by CiscoSystems
Some of these devices (most famously the WRT54G?) run a version of Linux, for which Linksys has made the source code available on their website. There was a lot of controversy about this a while back, because initially they did not release the source. Other devices run custom firmwares as far as I know.
Cisco 7xx
A Combinet product that was relabelled when Cisco acquired the company. Runs Combinet OS (yuck). It is no longer available and has been replaced by the Cisco 8xx.
Cisco Catalyst 19xx
Menu Driven. The enhanced edition runs a watered down version of IOS.
Cisco Catalyst 30xx
Menu Driven.
Cisco Catalyst 2926
Runs on CatOS. The other 29xx XL switches are different and run on IOS.
Cisco Catalyst 50xx and 55xx
Runs on CatOS. The Route Switch Modules run on IOS.
Older Cisco Catalyst 4000 and 6500 models
You used to have a choice of IOS or CatOS at install time. Earlier models had Route Switch Feature Cards that ran on IOS as a separate entity.
Earlier versions of Cisco Aironet software on the 340 and 350 series
Some weird Menu Driven thing via terminal (yuck), but does have a good web configuration interface. Cisco have now released IOS for the 350 series units. The newer Cisco Aironet devices run on IOS (1200s, etc).

The Cisco PIX Operating System (OS) is also known as Finesse

The environment is IOS-like, though the IOS FireWall commands and the PIX FireWall commands are completely different.

Here is a link to the history of the PIX