GIMP - GIMP is a multiplatform photo manipulation tool.
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Pixlr - Pixlr® makes it easy to transform everyday images into stunning works of art.
Pixelmator - Pixelmator is an image-editing application for Mac and iPad.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
Paint.NET - Paint.NET is a free image and photo editing software.