Sumo Paint is a powerful image editor with a long-standing history as a fully capable Photoshop clone in the browser. Being designed for the browser, it's accessible on practically any system and loads much faster than most comparable desktop tools while still providing core commercial features like layer-based editing with layer effects, multiple built-in filter effects and color adjustment tools.
Layer-based editing and compositing (Darken/Lighten, Multiple, Burn, etc)
Layer effects (Drop Shadow, Inner Shadow, Inner/Outer Glow, etc)
Color adjustments (Brightness/Contrast, Levels, etc)
Filter effects (Blur, Distort, Edge Detection, etc)