This is an ultra-easy tutorial that can add a powerful effect to images.
Step 1
Start with an image. Mine is from a friend over at deviantart.
Step 2
->
Create a new layer. On that layer, select the gradient tool and make a small straight line, like above.
Step 3
On your gradient layer, lower the opacity like above (70% worked good for me, you can experiment if you wish)
Final Product
Mine looked like this. Its a pretty simple tutorial, so if you have any questions or comments, please post below!