Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
PixBuilder - Free Photo Editing Software for PC
GIMP - GIMP is a multiplatform photo manipulation tool.
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.