I guess it depends what you're doing.
This is what I have installed: -
DevPac and MonAm - Not often paired with AMOS development, but AMOS and MonAm together can be quite handy for hacking around in other apps.
Brilliance 2.0 - The very best pixel editor for the Amiga.
CygnusED 3.5 - Nice, fast text editor.
Commodore Native Developers Kit for OS3.1 - Essential reading, if nothing else.
DevPac 4.12 (CU Amiga magazine coverdisk edition). Best file manager ever.
SnoopDOS 3.0 - Excellent util for debugging non-working software / missing libraries.
Enforcer - Pings up on illegal memory accesses and catches them, preventing potential Guru's. Handy if you're poking around with Assembly.
Grab a hardware reference manual as well. Whether you want to get that low-level or not, reading about screens, bitmaps and sprites should at least give you a clue as to how some of the more basic effects are created. Stuff like shadows and transparency.