Zen Brush!

Krita is a complete creative sketching and painting application with advanced, commercial quality features. It's free, open-source and available across all major desktop platforms.

MyPaint is a fast painting program with an emphasis on full-screen use. It lets you focus on the art instead of the program. You work on your canvas with minimum distractions, bringing up the interface only when you need it. MyPaint supports pressure or tilt sensitive graphic tablets and comes with an easy-to-use brush collection. There is a complex interface for creating own brushes, focusing on brush dynamics (changes with speed, pressure, randomly).

Paint (formerly Paintbrush) is a simple, ultra-basic graphics painting program that is included with almost all versions of Microsoft Windows. It opens and saves files as Windows bitmap (24-bit, 256 color, 16 color, and monochrome, all with the .bmp extension), JPEG, GIF (without animation or transparency), PNG (without alpha channel), and TIFF (without the multiple pages).

Clip Studio Paint, previously Manga Studio or ComicStudio in Japan, is a family of software applications for Mac OS X and Microsoft Windows used for the digital creation of comics and manga.

Seashore is an open source image editor for Mac OS Xs Cocoa framework. It features gradients, textures and anti-aliasing for both text and brush strokes. It supports multiple layers and alpha channel editing. It is based around the GIMPs technology and uses the same native file format.

Corel Painter is a raster-based digital art application created to simulate as accurately as possible the appearance and behavior of traditional media associated with drawing, painting, and printmaking. It is intended to be used in real-time by professional digital artists as a functional creative tool.

A simple but lovely natural media painting and sketching program. Art oriented, but capable of loading/saving photoshop files. A very cheap alternative to Painter, with a stripped down, elegant interface.

Drawpile is a drawing program that lets you share the canvas with other users in real time.

Verve (Dictionary: Energy and enthusiasm in the expression of ideas, especially in artistic performance or composition...) is the result of a curious idea I had to try using the graphics hardware to run a fluid dynamics simulation. Despite some little flaws, as you may observe at faster motions, it works astonishingly well across rather large resolutions. This permits highly detailed simulations of all kinds of liquids, and it screamed to be used for painting.

GPS (aka Gimp Paint Studio) is a collection of brushes, patterns, color gradients, accompanying tool presets and dynamics. Tool presets are a simply saved tool options, highly useful feature of the GIMP.

Livebrush is a drawing application. It employs an easy-to-use brush tool that reacts to your gestures through the use of velocity and other motion controls. By combining simple motion controls with customizable brush styles, Livebrush offers a fun and unique way to create graphics. You can install Livebrush on Mac and Windows though it requires Adobe AIR.

JugiPaint is a painting software with diverse sets of tools for various types of illustration work. It provides many standard digital painting features like layers, brushes, selections, transformations, etc., as well as some less often like guide layers and panels.