Assemblers

Any Z80 assembler can produce code suitable for the Next. However, the Next and most emulators cannot load raw blocks of Z80 code, so a Spectrum specific tool will be useful for creating .TZX or .TAP files with the necessary loaders.

Zeus-ish

Provides a complete Z80 IDE and Macro assembler, scripted disassember plus an integrated Z80 emulator for a range of machines including partial Next support

Supports the Next opcodes directly

Supports remote debugging on the Next using ParaSys across a serial link

Supports MMU paging in the integrated emulator

Supports sprites (core versions prior to 2.00.26) in the integrated emulator

Pasmo

A long established Z80 assembler, but has been out of development for a long time

Supports all currently known Next extension opcodes through this modified Pasmo from Russ McNulty and Tony Thompson and also now supports outputting .sna files to use with CSpect, thanks to Russ McNulty

SNasm

Included with the CSpect emulator

Supports the Next extension opcodes directly

z80asm

Part of Z88dk

Supports the Next extension opcodes directly, linking assembler with large z80 library, targets any memory configuration

sjasmplus

z00m's fork

Supports all (core2.00.28) Next extension opcodes directly and it is under active development (feedback is welcome).

Open source project ("BSD-3-Clause" license), windows executables available at releases, mac and linux users are expected to simply build from source (both make and CMake are supported).