Pixaflux is an free node-based image compositing application, designed especially for texturing 3D models. It has a huge variety of tools, ranging from noise generators, to mathematical operations, to model edge detection. It is available for Windows.PixaFlux Website
PixaFlux’s main interface is a node graph. It is an intuitive system that is used in many compositing programs, and PixaFlux works as you’d expect. The responsiveness of the UI is not the best, however. Often, pipes jump to the wrong socket in unexpected ways, and overall the node graph requires more pixel-perfect mouse movements than other programs.