Some syntax highlighting rules for coding in Z80 assembly as a Textmate language plugin. It should work in Sublime Text 2, or in other editors that support Textmate plugins. The syntax is particularly designed for RGBDS and Gameboy-specific Z80 instructions.
To install for Sublime Text 2 on Windows, copy the "Z80 Assembly" folder to here:
%APPDATA%\Roaming\Sublime Text 2\Packages
Enjoy!