An excellent tutorial from Matt Gemmell that teaches you how to add texture to UI elements.
Texture is an inherent part of any surface in the real world, and provides a cue to our brains that we’ll be able to grip or otherwise interact with that surface. Delicate use of noise can add a welcome note of reality to your interfaces, and make people want to use them.
You can see texture in action on a few sites that I maintain: here, here, and here.