I'm just starting some Cisco training and I would like to know if there are any freely available simulators available.
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
Zypher mentioned GNS, but I actually found it be a little bit flakey last time I tried it. I recommend you go straight to the command line text interface of dynamips called dynagen. On their page is a good tutorial on how to use it, it might have a slightly steeper learning curve, but works better in the end I think.
If you do go with GNS I wrote a tutorial here to which I never wrote the second part, but the second part isn't really needed.
For all these you are going to need the IOS images which Zypher mentioned (Which we can't tell you how to get if you don't already have a valid Cisco service contract).