Just looking at the textures is - in general - enough to see whether or not they are tiling seamlessly. I spent some time on the site, and as far as I can see none of the textures are seamless.
Using The Gimp you can make a texture seamless in a very simple way by using the command Filters --> Map --> Make seamless. However... this doesn't always give the desired result, as the example shows.
Hans
BEFORE
AFTER
