Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Material Maker - Cross-platform, procedural texture generation tool.
Grafx2 - GrafX2 is a bitmap paint program inspired by the Amiga programs Deluxe Paint and Brilliance.
Substance Designer - Substance Designer is a node-based non-destructive application for material authoring.
Piskel - Piskel is a website where designers online create sprites or pixel art.
TextureLab - Free, Cross-Platform, GPU-Accelerated Procedural Texture Generator.