From the website:

Paint.NET is free image and photo editing software for PCs that run Windows. It features an intuitive and innovative user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. An active and growing online community provides friendly help, tutorials, and plugins.

From the website:

Blender is a free and open source 3D animation suite. It supports the entirety of the 3D pipeline—modeling, rigging, animation, simulation, rendering, compositing and motion tracking, even video editing and game creation. Advanced users employ Blender’s API for Python scripting to customize the application and write specialized tools; often these are included in Blender’s future releases. Blender is well suited to individuals and small studios who benefit from its unified pipeline and responsive development process. Examples from many Blender-based projects are available in the showcase.

Kaleidoscope is one of the world's best tools for spotting differences in images and text, and now it supports merging of files and folders, too. Kaleidoscope integrates directly with Git, Subversion, Mercurial, and Bazaar to fit perfectly in your workflow.

Mac and Windows tool for taking multiple screenshots, annotating them, and combining them into a single document.

Windows tool for annotating, editing, and sharing screen shots and images.

Skitch is a free screenshot editing utility for OS X, iOS, Windows 8, and Android.

The app permits the user to add shapes and text to an image, and then share it online using Evernote. Images can also be exported to various image formats.

Originally developed by Plasq, Skitch was acquired by Evernote on August 18, 2011.

Web-based image editing with special effects and image manipulation features that can import content from PhotoBucket, Facebook, MySpace, Picasa, Flickr, Phanfare, or Smugmug. A free account is needed to save images.

FotoFlexer also offers an Open API to provide a simple interface for external websites to use the editing features - no API keys, REST or SOAP.

Picnik closed on April 19, 2013.

Picnik was a simple web-based image editor that could import photos from Facebook, Myspace, Picasa Web Albums, Flickr, Yahoo Image search, Google Plus, and also has upload capabilities. It was acquired by Google in 2010.