Gimel Studio - Next-generation of the cross-platform non-destructive, node based 2D image graphics editor
Material Maker - Cross-platform, procedural texture generation tool.
GIMP - GIMP is a multiplatform photo manipulation tool.
PixaFlux - PixaFlux is a node based image processing application.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
Filter Forge - On the surface, Filter Forge is just a Photoshop plugin, a pack of filters that generate textures...