I remember using a Z80 assembler on a CP/M 1.x machine, way back when. If it wasn't by DRI could it possibly have been (shock, horror) Microsoft??? We did have a Microsoft Fortran compiler, which was crap, but that was mostly down to being floppy disk based.

Not trying to be funny, I used the assembler a lot, but I really can't remember who supplied it.

Oh, just had a thought - this was on Research Machines 380Zs, so perhaps it was Research Machines home-grown one?