GIMP is a free & open source vector-based image editing software available for Linux, Mac, & Windows. It is often compared to and suggested as an alternative to Adobe’s Photoshop program.
GIMP – The Free & Open Source Image Editor
GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software, you can change its source code and distribute your changes.
Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done. You can further enhance your productivity with GIMP thanks to many customization options and 3rd party plugins. — from GIMP home page
Next Open Source Tuesday will feature the FreeBSD project.