GIMP

Object: GIMP
 * aka The GIMP

Howtos

 * Howto make a transparent PNG with The GIMP
 * Howto add the effect of a graduated neutral density filter on a photo using the Gimp graphics editor
 * Howto add the effect of a graduated color filter on a photo using the Gimp graphics editor

Related Wikis

 * Wikipedia: How to use the GIMP
 * External: The GIMP Wiki
 * Wikia: Computer Graphics Wiki