Ooh here's one for the serious tech-head. Or for anyone considering writing an emulator from scratch (that includes me, but just for fun. Ahem). The latest update to the Official Z80 Support Page is a presentation showing the internals of the Z80 CPU as block diagrams. It shows the active lines and micro-instructions during each T-State of instruction execution. But only for 4 asm instructions, and an INT request. The latter is most useful I think. But that's just me.
The presentation is available to download in Powerpoint format, natch!