Vyatta

What is Vyatta?
It is an open source, linux based software that provides fire-walling, routing, VPN's and intrusion prevention; in addition, to load balancing and other features found on CISCO routers
http://www.vyatta.org/downloads

Why Vyatta?
Free and open source is always good.
The beauty is that one can have the functionality of a close to CISCO router, but the difference being that Vyatta is open source and based on linux, while most CISCO routers will cost you some dough.
There is also an option to install Vyatta if one is so inclined.
Furthermore, the nice thing about the Vyatta implementation is that it can be used in a network lab for certain pen-tests or setup up in a virtual environment.

I hope they will add an implementation that one could just use to flash their firmware as in DD-WRThttp://www.dd-wrt.com/site/index or Tomato http://www.polarcloud.com/tomato , however, for more security related tests and some functionality similar to CISCO routers this is a good way to go.For router service enumeration, fingerprinting videos, and some interesting stuff check out this
video:http://securitytube.net/Router-Hacking-Part-2-%28Service-Enumeration,-Fingerprinting-and-Default-Accounts%29-video.aspx

And if you have a DD-WRT, you might want to try some fun stuff with this:
http://security-sh3ll.blogspot.com/2009/07/exploiting-new-dd-wrt-remote-root-with.html