Colors! is the Nintendo DS app that most powerfully makes the point that homebrew is not just a wheedling cipher for piracy. Developed by Jens Andersson, it offers hard and soft brushes, pressure sensitivity, a 512x384-resolution canvas, and can send paintings as PNG files via email. It has a Corel Painter-style hue-circle, saturation-triangle palette.
Wired's How-To Wiki has a tutorial on how to work with it to produce stunning works of art; pictured above is a reproduction of a Rembrandt self-portrait by Jason R. Dunn.
Forthcoming versions of the app will have undo, a levels tool and collaborative painting.