Here's a quick Photoshop tutorial for creating a reasonably realistic looking wood grain effect. It's a quick technique that looks pretty good. Once you learn it, it can be accomplished in under a minute!
1. Create a new image. I'm using a 150x150px image, but any size will work.
2. Select a medium brown (I'm suing R140 G98 B57) and fill the image (Alt+Backspace).
3. Now go to Filter - Texture - Grain. Set the contrast to 50 and use "Contrasty" for the type.
4. Again, go to Filter - Texture - Grain. Keep the contrast the same, but this time, choose "Horizontal" for the type
5. Go to Filter - Blur - Motion Blur. Set the angle to 0 and the amount to 20.
6. Go to Image - Adjust - Brightness/Contrast. Set the Brightness to +50 and the Contrast to -25.
7. Go to Image - Adjust - Hue/Saturation and set the saturation to +40.
8. Hit Ctrl+L to bring up the levels dialog and set the input levels to 18, 100, 255.
9. Almost done, we just need to add a couple knots in our wood. Use the circular marquee tool to select an oval wherever you want a knot to be.
10. Now go to Filter - Distort - Twirl and use an amount of around 130, slightly higher, or slightly lower, depending on the look you want.
There you have it, (reasonably) realistic wood grain. It still looks somewhat computer generated, but... well, it is.
