WLA DX
From GbdevWiki
WLA DX includes the WLA macro assembler, the Binary to Text converter WLAB, the disassembler WLAD, and the WLALINK linker. WLA and WLALINK are both available for a variety of targets, of which the GB-Z80 was the initial target.
Distribution
http://www.villehelin.com/wla.html
Bugs
(Please add any bugs you might encounter to the list below)