OPENBSOD is one of free web-sites where you can find everything about networking and network virtualization.
Web-site presentation26decMore News
Virtual laboratory building
Certification exams practicing requires a lot of real hardware. It is a big expense. Using IOU-WEB, Unetlab and GNS3 you can design real laboratory environment for testing and studying with any topology that you need.
Here you will find a lot of IOS/IOL images, topology examples and useful literature. Have to note that most of downloads available only via Hyperboria network: