This guide covers all the current necessary Editors/Utilities.
Mapping
Weland
A cross-platform forge like map editor. No built in visual mode, maps must be textured using another tool such as Visual Mode Lua.
Visual Mode Lua
Texture maps from within Aleph One.
Atque
A cross-platform utility for merging and splitting map/image files.
Physics, Shapes, Sounds
Shapefusion
I'd recommend Shapefusion for editing Shapes, Sounds and Physics files. Largely replicates the functionality of Bungie's original tools, with a simple interface.
Physics Editor One
A cross-platform but limited physics editor written in Java.
Emulation
Redundant Robot
A tutorial for setting up a classic Mac OS emulator such as bassilisk or sheepshaver, with links to those emulators.
Textures
Aorta
A cross-platform utility for creating DDS textures for use in Aleph One.
GIMP
A cross-platform image editor.
Other Links
SBO Utilities Page
Another list of editors/utilities, some of which are not listed here.
Infinity Utilities
A list of older utilities and forge/anvil updates, most of which requires Mac OS 7 - 9, or an emulator.
Trilogy Release
Forge, Anvil and the Marathon Infinity Manual, which has instructions for using both those editors. Since Weland is very similar to Forge, reading the Infinity Manual should help with learning to use Weland.