The Brick Painter is a PHP web application that transforms any image (of a reasonable size) into bricks, for easy reproduction. Live version accessible at http://www.arpia.be/brickpainter/
It was initially created to allow the author to create LEGO-based reproductions of famous impressionist and surrealist paintings.
The Brick Painter has a limited colour palette - it only includes the colours that are available through the LEGO.com webstore as of October 2014.
The end-result does not (yet) take into account limitations of LEGO plate sizes (e.g. "Earth green" is only available from the LEGO.com store in 1x4 plates, not 1x1 or 1x2).
Peter Craddock and the Brick Painter are not affiliated in any way with any companies of the LEGO group.