The GIMP: a freely available graphics application.

(related variations: The GIMP GIMP)
The GIMP is one of the best image manipulation / composition applications anywhere; it stands for GNU image manipulation program. While it is true that PhotoShop can do some things the GIMP cannot, it is also true the GIMP can do some things PhotoShop cannot. The GIMP is available for pretty much every UNIX-like OS and Mac OS X.