There is only one file type that uses the .TEX0 extension:
Wii Texture File
Texture file used by Nintendo Wii games such as Mario Kart Wii; typically found within a resource package file (.BRRES) and contains textures that are used to display the surface appearance of 3D models.
You can open BRRES files as well as TEX0 files using BrawlBox.
Updated: February 27, 2013