Goran Devic has been building a Z80 for FPGA, aiming to implement at the gate level instead of the usual RTL level.

See various subpages at http://www.devic.us/hacks/ (broken: see now http://baltazarstudios.com/tag/z80/)
and code at https://bitbucket.org/gdevic/a-z80

(And see also relevant writeups by Ken Shirriff of his own findings on the Z80 internals: http://www.righto.com/search/label/Z-80 - Goran started his project on his own but is now following Ken's/visual6502's progress too.)