SyX created a Z80 definition file for the great Windows text editor Notepad++, so you can enjoy Z80 syntax highlighting in the editor now. It mainly supports the assembly syntax of VASM, but you can also add the MAXAM (or any other assembler) directives. You can read more about the the definition file and about some known bugs in the CPCWiki forums.
To install the language definition file, use “Language”, “Define your language” and “Import”.