Rgbds (Win) - Rednex Gameboy Development System.
Probably the most used assembler-suite.
Music Box (Win) - Gameboy Sound Editor
Coltuner - little gbc-program that helps with getting the colors of your images right on the real hardware
Pcx2gb (Win) - converts .pcx images to gameboy tile-format
REW (Win) - pretty good freeware win9x based gb/gbc emulator.