GIMP - GIMP is a multiplatform photo manipulation tool.
Material Maker - Cross-platform, procedural texture generation tool.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
Substance Designer - Substance Designer is a node-based non-destructive application for material authoring.
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.
Filter Forge - On the surface, Filter Forge is just a Photoshop plugin, a pack of filters that generate textures...