The GIMP is an image editor often compared to Photoshop and the like in scope and power. It's free, open source and cross platform.
One of the biggest complaints (especially from PS users) was GIMP's "lack" of a single window UI, favoring separate, free-floating dockable, pinnable elements instead. While single window mode is still not the default (select it under "windows" drop down on top bar) 2.8 DOES finally have it, and it works quite well. Other improvements have been made to this release including UI beautification, layer groups, speed enhancements, and more.
GREAT software!