Hardware:
A1200 with Blizzard 1230 IV 64 mb, 40gig HD, 22 speed DVD re-writer, PCMCIA network card and a Commodore SVGA dongle for double scan modes.
Software:
FrexxEd. Great editor with awesome scripting system (FPL, which is 99% C). You can make that editor do almost anything. Here's a simple assembler LVO completer I wrote in FPL (still needs some work, but is already usable):

Ace editor, and nothing holds a candle to it.
For the rest: A few assemblers. Currently switching to Barfly, and integrating it with FrexxEd. Adpro 2.5.1, Brilliance 2, WHDLoad, Directory Opus 5.82 (set up like Dopus4), HippoPlayer, and some other stuff.