Ok another very good tutorial from Pawel Filip blog (check his Deviantart page also)
This time its about how to equalize textures in Photoshop (can be done in various other 2d applications)
When you take photographic pictures of textures, the light is not the same in the four corners of your image.
That's bad because you can't make good seamless textures when you have different values of light.
Here's the link to the tutorial
And here's some examples, on the left its before, on the right its after the tutorial
(all images courtesy of Pawel Filip on his Tolas blog)