What are your favourite assemblers, compilers, environments, interpreters for the good old ZX Spectrum?
I always used to use Roybot Assembler - which had you enter your program using the BASIC editor and REM statements. It comes with a decent debugger/disassembler that lets you single-step machine code too.
The Hisoft Gens and Mons assembler and disassembler (aka Devpak) are probably fairly popular.
For high-level compiling, the Mira Modula-2 compiler is very good.