I've seen a few guides on here on how to make wood grain but I didn't feel they looked like wood enough. This technique was learned about 5 years ago and I've never forgotten it. Hope you don't too. :)
This technique was made in Photoshop CS2. I tried to make it in The Gimp but I couldn't find the same filters needed. If you can do it in Gimp, I would love to know.