Free GM Resource: CGTextures
One thing I find myself referring to often is my rather small collection of texture files. They end up being used in everything from maps to character portraits. It is easy to see how they might be useful in creating maps, but I also like to use them in Photoshop to dress up character portraits and paper minis. If my character is wearing leather armor I can just color it brown or I could color it brown, make that portion opaque, and then place some leather texture behind it.
CGTextures.com is a great source for texture files and just happens to have some good tutorials for manipulating those files for use in your projects. Definitely a good site to bookmark if you do any digital artwork. They are pretty free with the materials as long as you aren't reselling the digital files. Of course this is a generality and you'd want to reference the site's license and FAQ.