This effect was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
This filters supresses unwanted lightness variations in tiling textures. It is based on the idea of Peter Hajba, the artist behind Max Payne games and Future Crew demos.
In a Gamasutra article titled "The Power of the High Pass Filter" Peter described a way to make dark and light spots in tiling textures less repetitive. I've just implemented his approach in Filter Forge.
suppress flatten dampen brightness luma luminance homogenous homogenize
This filter has earned its author one of our rewards. They are given to the authors of popular filters and include a free copy of Filter Forge.