Seamless materials, you make them from seamless textures. And you make seamless textures in photoshop or gimp.
However there is some tiny but not cheap, but has demo. Filter for photoshop (can also work standalone).
It is called filter forge. You can make a bunch of nice seamless textures. However they are usually for web pages,
so if you want good game quality ones you need to work more on those from filterfroge. But they are great as base
for new texture or for development while you need some placeholder until you make real good quality ones.
Also with filterforge i would recommend crazy bump, or awesome bump for creating normal maps.
When you more advanced in mater of textures you can get alleghoritmic products.
To make textures for games i recommend:
alleghoritmic substance painted and designer ,
filter forge + photoshop or gimp ,