For my money, one of the most impressive projects is
Adam Dunkels'
multitasking OS for 8-bitters, Contiki. Amazing stuff; includes TCP/IP
and even Ethernet support on some hardware.
I'm actually working with an 8052 microcontroller port of his tcp/ip
stack for the embedded network device I've been designing. I'm using
it with an ISA nic and an 89C51RD2 chip from Atmel, should be finished
for the end of the month for an electronics competition called Bright
Sparks. Similar to
picnic.sf.net.
The port of it to the Magic-1 Homebrew CPU was really brilliant.
also, I got out a little book from the library on programming the z80
in assembler :)
-Arron