This tutorial was originally posted at psdtutsplus.com. The original article can be found here. This one is so easy and awesome, I had to post this one.
Take a blank document. Activate the Gradient Tool, in the option bar and choose Linear and Mode: Difference. Create a random gradient, from top to bottom, bottom to top, left right, and right to left. Just go crazy!
Go to Filter > Stylize > Emboss. Hit Ctrl+L (Cmd + L for MAC) and move the white and black sliders to the middle. And Voila
You can color correct after this to suit yourself. You may add a Pattern Overlay with some paper pattern to give a better Texture.