GIMP - GIMP is a multiplatform photo manipulation tool.
digiKam - Professional Photo Management with the Power of Open Source
Fiji - Fiji: A batteries-included distribution of ImageJ.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
ImageJ - and Other Health Care