TextureLab - Free, Cross-Platform, GPU-Accelerated Procedural Texture Generator.
Substance Designer - Substance Designer is a node-based non-destructive application for material authoring.
NeoTextureEdit - NeoTextureEdit is an open source (GNU LGPL v.
Filter Forge - On the surface, Filter Forge is just a Photoshop plugin, a pack of filters that generate textures...
GraphicDesignerToolbox - Graphics editor for composing textures, buttons, logos, icons.
PixaFlux - PixaFlux is a node based image processing application.