C program notes, dated from 1995 but very clear and at simple level explaining the syntax you'll need to know.

Home of the z80, massive site with many links to Z80 CPU resources

The following are links to other addin developers sites...I don't intend to try to rate or order them, just list them:
Bertus Beer Source code
Dimitriy Source code
John del Ferro Source code
Chris Harris Source code
Mr. Idogawa Source code
Gerd Krahmann Source code
Marc Nijweide
David Riley (Irish_iiii) Source code
Shai Seger Source code
Daniel Schmidt Source code
Amiram Stark
Alan Tam
Marcel Timm (Mantillo)
Dataslim2 sources

Link to rex technical sites other than addins developers
Peter Kong
Paul Lenz
Peter Richard
Marek Wodzinski