The best Z80 assembler for Spectrum from 1988, PIKASM.
Info by H. Med

I believe it can read source texts from GENS (and save them in own compressed
format), and is compatible with GENS even in compiler options (though it has
some more). It compiles very fast and knows the undocumented instructions.
Controls are fairly straightforward , you only need to know that instructions
must be in uppercase, a line is inserted with CAPS+9 and to the menu you get
when you press both shifts (=extended mode).  As an example there's my little
program, an Art Studio - like arrow controlled by QAOPM or Kempston joystick
(use extended mode, G to load it, then A to compile, then R to run) . Pikasm was
written by Tomas Rylek (mostly?), Petr Vones and Mirek Fidler.

Perhaps more useful for Spectrum emmulators are the debuggers.  The first one,
VAST, works in screen memory, so you can use all your 48k for the game you are
hacking. You can load VAST normally as SCREEN$, that is LOAD "VAST" SCREEN$ :
RANDOMIZE USR 16384, or, load it at any location in memory and run it there (it
will relocate itself to screen). VAT is similar, but runs in normal memory where
it's loaded (useful for 16k games!). These programs are very MONS-like and have
some features MONS lacks. They know the undocumented instructions, handle R
register correctly, can list a program in basic (key k), step not only one
instruction (:), but take CALLs at once (pound sign), find text strings in
memory, find with a mask..  From most modes you can escape by $ sign or with
CAPS+1.  VAST and VAT were written by Tomas Rylek, there are some additions by
Patrik Rak (CAPS+J loads a headerless file by IX,DE and A, CAPS+S saves, and in
VAST there's some key with CAPS that lets you change the third of screen it
uses.)

All the programs are to my knowledge freely redistributable.