This is a great effect for images the express alot of colour, its also really good for forum signatures. You have most likely seen them on forum signatures anyway. Well this is how you do it!
This is what im on about you see, it's obviously a dark picture so its not the best example.
[Tutorial]
1. If your creating a signature, i would suggest 400 x 125 pixels. If your using something else then ignore this.
2. Create another new image 10 x 10 pixels with it set as transparent. Just like this:
3. Right click your brush tool and use your pencil tool to draw a diagonal line from top left to bottom right like so:
4. Go to Edit > Define pattern > Save as pattern 1 (Or whatever you like)
5. Create a new layer on the image you want the gradient on.
6. Right click the stamp tool and use the pattern stamp tool to create the effect across the image of your choice.
7. THATS IT! Your done!