This is a simple graphical frontend for Nvidia's nvdxt comandline tools. There is only a german version, but i think it's ease to unterstand for everyone. The download includes a older version of Nvidia's nvdxt comandline tools (it works fine) but you can update by coping the latest files in the program directory.
you can download DXT Tool v1.0 here:
http://www.s-c-o-r-e.de/maptut/mod/002.htm