Being a web designer, I tend to collect sites with resources, here are a few:
This site has some nice free ones, http://texturemate.com
If you only want a pattern instead of a texture, http://patterncooler.com is good.
As far as paid programs go, Filter Forge (http://filterforge.com) is pretty nice and they have a demo for you to test it out.
Hope those help! Game Textures looks awesome too