RAWTex is a save plugin for GIMP 2.x allowing to create raw textures for the PC, PSP, NDS, PS2 or any other architecture handling chunky or compressed dxtc formats.
It can save raw RGBA (5650, 5551, 4444 or 8888) & indexed (T4, T8) textures (with 5650, 5551, 4444, 8888, 0333, 0444, 0888 or X68000 palette modes support) as well as DXTC (DXT1, DXT3, DXT5) compresssed ones, it supports the PSP textures swizzle mode and can generate mipmaps, it also have a color key feature to handle transparency and can convert grayscale pictures into alpha channel only textures.
Note:
Work is been made to include support for Amiga & Atari ST planar exports (only chunky is available for indexed modes).