Nadeo is also a nice example of mod support, they haven't released tools, because they simply aren't needed in their games.
Car meshes go as .3ds, character meshes go as .fbx, textures go as .dds or .tga, animated textures go as .bik, and sounds go as .ogg, not packed in anything like in most other games, actually custom creations get loaded through the "ManiaPlanet" (for ShootMania and Trackmania 2) or "Trackmania" (for Trackmania Nations/United Forever). Things such as dirt marks, buttons, etc go through the ManiaPlanet map (or through the SteamApps/common/...), or the Trackmania Nations/United Forever map in the program files.
Custom scripts (for weapons/maptypes/gamemodes) go as .Script.txt through the Documents/ManiaPlanet directory,
So, if you have programs like Paint.NET, notepad, 3dsmax (or blender with plugins), some tools to convert stuff to .bik, and an .ogg converter, you can mod say... 93% of the entire game.
ADD:
In their latest update they've added an importer for custom made character/block meshes too, they're making it too easy for us.