Besides the Applications included in the OpenMW source tree, there are various external tools available to assist with OpenMW development.
The vanilla Morrowind game executable. Perhaps more of a game than a tool. But we list it here as a tool because it should be used as such during development. Before reporting OpenMW bugs, check to see if the same bug exists with the vanilla Morrowind executable. Use vanilla Morrowind to research game formulas, or test for OpenMW regressions.
The game's assets (meshes, textures, etc) come packed in .bsa files. You will need a special extraction tool to inspect these files. BSATool included in the OpenMW code repository can be used to do this.
BSATool is a command line application, see the bsatool --help for usage instructions.
NifSkope is a free and open source tool used to inspect and modify .nif files. Morrowind uses NIF files for its meshes and animations.
NifSkope is cross-platform and uses QT, you should be able to compile it on your platform.
TES Construction Set
The original game's construction toolset used to create and modify game content.
On Linux, WINE runs the TES Construction Set without issue save for one problem: you can only save to existing files, not create new files. To workaround this, use MWEdit to create a completely blank .esp you can then modify in TES-CS.
A command-line inspection tool for ESM/ESP game content files. Very useful to quickly dump a specific record from a file, search for occurrences of a string, or compare two files.
ESMTool is a command line application, see the esmtool --help for usage instructions.
ESMTool is included in the OpenMW code repository.
OpenMW-CS is intended as a cross-platform replacement for the TES Construction Set, that also comes with many new features. At the time of writing, OpenMW-CS is still in alpha stage, but can already be used to create simple plugins. Terrain editing and cell editing is still missing though.
OpenMW-CS can create plugins compatible with the vanilla Morrowind engine, just rename the output file from .omwaddon to .esp and they should work.
OpenMW-CS is included in the OpenMW code repository.