Are there any good free Cisco router simulators out there?

GNS Is the best one out there that i've been able to find. You'll have to get your hands on your own IOS images though.

I was once told of someone using it as a production router ... don't know why but they were.

if you can "find" **cough* * * cough * a copy of Packet Tracer out there, it will allow you to directly configure the routers through IOS, as well as add or remove modules from the hardware. not to mention it includes more than just routers, you can design logical and physical networks, that connect, and send packets... its crazy, check it out

http://www.cisco.com/web/learning/netacad/packet_tracer/packet_tracer_03-4_web.html

You can try zebra or quagga (zebra's fork), with cisco sintax but not all cisco ios features.