Open Source Cisco Certification Study Aids?

phee asks: "I'm trying to get my CCNA. I have some Cisco hardware (a PIX, switches, T1), but no routers to play with. Router knowledge is a big part of the CCNA exam, so I've been looking for router simulators. All the ones I can find are very commercial and very Only-For-Windows(tm). Anyone know of any open source simulators that work in Xwindows or text mode even? There's just no substitute for hands-on experience... " If such simulators don't exist, how about online study-guides or Web pages that talk about such information?