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.
Step 1: Open a blank window
Lets start off by opening a blank window of any size u want. I used 800x600. Then, change the foreground and background color to a dark brown and light brown color.
Step 2: Make a cloud
Now go to the top menu and select "Filter > Render > Clouds."
Step 3: Add the Grain
To give grain, we are going to the top menu again and select "Filter > Distort > Shear." To make it curve just click and hold on the graph, and drag it toward it's edge. Keep doing this until your Zig-zag line is just about similar to mines. (You can more points if you would like)
Step 4: Rinse and repeat
I'll assume that your version of Photoshop has a filter repeat option by pressing "Ctrl + F." Press this combo to add more zig-zag-ness to your image. Keep doing this combo until you are satisfied with the grain.
If you don't have the filter repeat option (unlikely) then repeat step 3 three of four more times.
Step 5: Final touches
Now we could just stop at step 4 but I wanna go a little bit further. If you want to change to color of the wood, you could just go to "Image > Adjustments > Levels" and move the three sliders until you get a desired color.
Finally, I want to make the wood seem more "hard." We're just going to "Filter > Sharpen > Unsharp Mask" and add a subtle sharpness to the grain. My settings were: Amount - 79%, Radius - 0.9 pixels, and Threshold - 0 levels.
Well, there's our finished woodgrain. I hoped you liked my 1st Instructable :D