Grimrock

I'd like to point to the free music archive (http://freemusicarchive.org).

It's where I got the music for my latest mod.
The quality is good, but be sure to check the conditions it is released at.
Almost all I saw where free for non-commercial use and only required to credit the author of the music piece.

I have updated the master list with all of your great suggestions. Thanks guys - some nice sites there that I didn't know about. Will hopefully be of use to others, especially those that are just getting into modding.

I'm just dipping my toes into modding at the moment and found http://crazybump.com/ quite a cool bit of software that will generate your specs and norms for you

I found and tested Paint.NET http://www.getpaint.net/

This is free painting software with full compatibility with dds files.
(I tested it for very easy for recolorize flame particles to another color.)

save setting:
A8R8G8B8
Generate Mip Maps
Supersampling