Windows/Linux: MyPaint is an open-source image editor that allows you to edit images in fullscreen on a limitless canvas — unlike the fixed canvases found in traditional editors. MyPaint also supports pressure-sensitive tablets, for those sporting tablets and touch screens.
MyPaint includes a wide array of extensively customisable brushes. There is also basic layer support, colour history and a colour palette that includes complimentary colours. Unfortunately, copying and pasting into external image editing programs isn't supported but you can save your images in common formats like PNG to import them into other image editors.
It won't replace Photoshop if you need a feature-rich program but — as essentially a simplified Corel Painter — it offers simple and intuitive painting. MyPaint is open-source freeware for Windows and Linux systems only.