CoCoIO is based on the WizNet 5100s ethernet chip. This amazing device can manage 4 separate TCP or UDP network stacks all by itself, the host computer's only job is to feed these pipes.
Some applications can be written in basic! I'm describing a simple web browser in the blog section of this site. Major update coming soon.
Which is fun, but we need real software, and at least two talented guys have stepped up. Mike 'n6il' Furman is well on his way to drivewire over ethernet, and the mysterious 'strickyak' is writing the rest of the BSD socket (things like arp,dig,ping,dhcp,ntp,tftp). Try these links:
https://github.com/n6il/cocoio-dw
https://github.com/strickyak/frobio
https://github.com/rickulland/CoCo-www