For the Z80
http://www.z80.info/
The 8080 is the same as the Z80 (indeed, Zilog built the Z80 to be a direct, but more powerful, replacement for the 8080).
There's a fair amount of interesting stuff here for the 6800, 6809, and 68HC11
http://www.programmersheaven.com/zone5/cat26/
The truth of the matter is, though, if you learn the 6502 you pretty well know the 6800, because the 6502 was designed by some of the same engineers who did the 6800. In many ways, the 6502 is streamlined 6800. The 6809 is a souped-up 6800, and the 68HC11 is more of the same, IIRC (it's only been near 20 years since I've worked with them
). I think the 68HC05 is part of the same family as well.
The Zilog Z8 can be found at
www.zilog.com. It is its own little chip, rather like the PIC, but is completely register oriented (all of the
RAM is the register file).