Bunch of GIMP stuff. Has a lot of anything and everything GIMP but not specifically sigs. http://www.gimptalk.com/forum/gimp-tutorials-and-tips-f8.html
DeviantArt... bunch of stuff, some good some bad http://browse.deviantart.com/resources/tutorials/appreference/thegimp/
Here is 3 large pics for how to get a specific sig style.
http://hosting02.imagecross.com/image-hosting-02/7388code-chozo-tut-part-1.png
http://hosting02.imagecross.com/image-hosting-02/8457code-chozo-tut-part-2.png
http://hosting02.imagecross.com/image-hosting-02/5405code-chozo-tut-part-3.png
If I may I would suggest
Kimi wa Boku no Niteiru by See-Saw (Gundam SEED Destiny ED4)
Prototype by Ishikawa Chiaki
Or you can use Colors like the last ep for Code Geass R1
Well those are nice choices,but what I have in mind is a sad ED theme for the Project and I have pulled out of my folder 18 of the most saddest ED themes I've ever heard