Current version: 1.0.1
Requires MPW and MPW IIgs Assembler
MPW AsmIIgs Formats is an add-on for Apple's Macintosh Programmer's Workshop that adds support for syntax coloring when writing Apple IIgs 65816 assembly language. To use this, you need a copy of MPW and a copy of the MPW IIgs Assembler (available for purchase through Syndicomm).
While MPW was historically available free of charge directly from Apple, they have removed this "obsolete" software from their site. You can still find it on various mirrors, including this one.
To use MPW IIgs, you'll need at least the file MPW-GM.img.bin; this is the complete MPW package, in its final release form.
This file is a MacBinary-encoded Disk Copy image file, so you'll need a utility that can strip the MacBinary wrapper; StuffIt Expander, for example, can do that for you.
A better option, nowadays, is to use Kelvin Sherlock's awesome mpw tool for Mac OS X. This is a Mac command-line utility that incorporates a 68k emulator, and emulates all the classic Mac OS traps and MPW routines needed to run the MPW IIgs tools (as well as many MPW tools). This is a fantastic way to do Apple IIgs development on OS X, since you can use it with your favorite Mac editors, including BBEdit or Xcode! Note that when you do this, you don't need these formats at all, since you're not using the MPW editor.