I have found a number of tools useful:
CA65 (https://www.cc65.org/) I like this assembler as it can produce a proper output listing, I haven't used any of its clever features yet though I've just used it as a simple in-line assembler to recreate Acorn ROM's having disassembled them with DA65.
Notepad++ https://notepad-plus-plus.org/ my favorite editor.
HxD (https://mh-nexus.de/en/hxd/) - I mainly use this for comparing ROM files and converting binary files to Intel HEX.
TL866CS EPROM Programmer - well priced little programmer, has a couple of issues: low Vpp voltage and can't program 2732's, here's how I fixed that TL866 Adapter.
GBS8200 RGB to VGA Converter - not really a dev tool but a great bit of kit that means you don't have to lug a PAL monitor around, I have a boxed version and one on a Eurocard GBS8200.
My 2114 RAM Tester 2114 Tester