Separate assembler / disassembler or something else completely?
The most powerful disassembler - IDA supports z80.
Also a list of disassemblers published in the Software Development Tools for the Z80 Family
It depends on what operating system you are running on; there are many good tools here:
http://www.z80.info/z80sdt.htm
The first program I ever wrote was in the Z80 assembly language.