A free assembler/linker package for the Game Boy and Game Boy Color (source files)
rgbds_source-0.9.4-1-source
RGBDS' four programs cover the whole compilation pipeline into a ready-to-play ROM, from RGBGFX (the image converter) to RGBFIX (the header "fixer").
Each tool can easily be swapped for a custom one, or used stand-alone as part of another process.