Because I wanted to learn more about ethernet and TCP/IP networking, and because I was dissatisfied with existing TCP/IP implementations for the Apple, I decided to write my own implementation. It is called Marina.
I started on February 1st of this year, taking a Uthernet/CS8900A driver that I'd written about 2 years ago, and in 2 months' time I have written decent implementations of IP, ARP, UDP, DHCP, with little bits of ICMP, and the start of a DNS resolver.
As far as I know, Marina is the first IP stack for Apple II to include a Link-Local addressing implementation. It has several other unique features too, but that was the one feature that I really wanted to have in a TCP/IP stack for Apple II.
Visit the Marina web site for documentation and downloads.