Material Maker - Cross-platform, procedural texture generation tool.
GIMP - GIMP is a multiplatform photo manipulation tool.
Filter Forge - On the surface, Filter Forge is just a Photoshop plugin, a pack of filters that generate textures...
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
TextureLab - Free, Cross-Platform, GPU-Accelerated Procedural Texture Generator.
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.