Paint.NET
Strong candidate as a potential substitute for Photoshop for beginner/intermediate users.
FastStone Image Viewer
One of the best image viewer, converter & editor bundles.
IrfanView
The Swiss Army knife of image viewers/editors. Extremely popular.
Google Picasa
Powerful photo manager software from Google. Includes 1-click photo retouching features. Easy online photo sharing.
GIMP
Very capable graphic editor with Photoshop-like features. Often labeled as free Photoshop alternative.
PhotoScape
Another good free and easy-to-use photo editing application.
Inkscape
An open source vector graphics editor that uses the Scaleable Vector Graphics (.SVG) format.
RIOT – Radical Image Optimization Tool
A free tool that makes light work of optimising images for the web while maintaining quality and small file sizes.
XnView
A small utility for viewing and converting images that reads around 400 formats.
XnConvert
Easily batch convert, resize and apply effects to images with this open source software.
Zoner Photo Studio Free
Tweak, fix, manage and share your precious memories with Zoner, software that rivals paid software.
Neox Screen
A free and open source screenshot manager that’s lightweight yet powerful.