I’m sure you’ve all heard of Cisco IOU by now, and I’m finally catching up with the other bloggers of the world by mentioning it. It’s an executable version of an IOS image that runs on a Unix (or Unix-like) platform and it’s the backend behind Cisco’s Learning Labs.
Be sure to check out PacketLife.net if you want to do some labbing with Cisco gear.
Check out GNS3 Vault if you haven’t already. It’s worth the time.