In the process of working on my Titanic Map, I came across two AWESOME free pieces of software (for windoze, sorry linux and mac users...). The first is Genetica Viewer. Genetica Viewer comes with a whole bunch of preset tileable textures which are almost completely editable. The editing is done via changing the "Seed" and "size" of the texture. Even though the type of textures that you can generate is somewhat limited, they do have a really large selection of textures and you can make each one unique by messing with it. They also will render a height map along with the image so making a bump map is quite a bit easier.
The second (and my personal favorite) is Wood Workshop. Finding really nice tileable wood textures seems to be a pretty big issue. However This program "eats seamless wood textures for breakfast." Not only does it have quite a few wood types and colors to choose from, it also has the ability for you to add weathering, tile squares, and mess with every part of the wood texture including the knot strength, the exact colors of the rings, and pretty much every single part of the wood. Basically you can make EXACTLY the wood texture that you are looking for.
Both of these programs allow everyone to use the textures they make for commercial and non commercial use, and you can even redistribute as long as you note where to download the software that you made it with. And again, both of these programs are completely free so its not like you have to pay to get full use of the textures. And there are no watermarks or anything either. I still haven't found a downside.
In short, I would Suggest EVERYONE (who can run windows) to at least download and mess around with it to see if you like it, because it has proven to be extremely useful to me!