I just saw this on Machinima.com, Spiral Graphics has released a new seamless texture pack with about 150 materials (they have two older packs available as well). You can get them from http://www.spiralgraphics.biz/packs/
The catch is that although the packs contain JPEG previews, the materials are in the GTX procedural texture format which I am not familiar with. I think this release is mainly to promote their Genetica texture editing software so I am not sure how useful they will be to Blender users.
How easy/hard is it to work with GTX files?