Any Free Simulator ...

To tell you the truth, if you are serious about learning about the cisco routers, it'd be best to pick one up and just play with it.

Although its not all that useful for all the latest features, and is really slow, a cisco 2501 was the workhorse CPE router for the longest time and is probably $30 to $50 on eBay.

Then you'll get to do things like try out password recovery or whatnot that a simulator isn't going to cover (although I suppose that is a bad example, as this router is old, and different than any current router for that function).

The simulators generally only lead you in the way that the lesson is teaching you, and don't support the whole range of commands, usually a subset of what they want to teach you for a given function.