Seamlessly repeating or tileable patterns are useful for a variety of things from backgrounds to texture maps in a 3D application. Essentially the idea is to manipulate the image so that when it is repeated in a tile pattern, there are no visible seams or obvious areas of repetition. Choosing the proper start image is very important, because the surface should not contain any perspective or lighting changes. Then go to Filter>Other>Offset. Make sure the Undefined areas a reset to Wrap Around and use the horizontal and vertical sliders to shift the image so the seams are clearly visible. Then use retouching tools, such as the Spot Healing Brush and the Clone Stamp tool, to remove any unsightly seams. The result will be a file that can be tiled across any expanse of digital space and you never have to worry about seeing a hard edge!