TexSynth
This little program is meant to help you generate tileable textures. But it takes way too long. I had two attempts. The first with a complex texture the second with a abstract version of potato skin. Both images were taking over 1/2 hour to do. It is faster to use the offset filter in Photoshop, or the equivalent in the Gimp (PSP 4 doesn't have offset filter and it's tileable function is pretty naft, but all you do is select the entire image, cut it out and paste it back in as 4 corners in reverse. ie bottom right corner/quarter moves to top left; bottom left corner/quarter to top right, etc. then use the clone brush to remove the lines. Repeat until seamless - takes maybe 10 minutes)
Overall Don't bother 1 out of 10
Overall Don't bother 1 out of 10