Good article with introductory discussion:
Cisco 7200, Cisco 3600 series (3620, 3640 and 3660), 3700 series (3725, 3745) and 2600 series (2610 to 2650XM, 2691).
Dynagen/Dynamips (excellent choice and good for windows and *nix):
Boson (proprietary at least 100USD$):
pyios (not as much following as the other two):
As a general word of caution, if you have no experience with Cisco or other CLI based router admin (such as linux and red hat derivatives), I would not recommend diving in unless you have a lot of spare time on your hands in the event that you accidentally delete all routing tables or cause other serious problems!
Even in a professional business environment, due to some differences between IOS and hardware versions and the need to reboot for some settings to take effect, it’s always a good idea to perform changes after hours when critical business operations will not be interrupted by internet or service downtime.
Official Cisco IOS Manual, http://www.cisco.com/en/US/docs/ios/12_3/featlist/sec_vcg.html
Manipulate NAT and other general help references and top hits on Google for common Cisco beginner questions: