GIMP

GIMP (GNU Image Manipulation Program) is an open-source image application software, available for several operating systems.

Main features

GIMP has all the major features of non-free image processing software. It is also extensible through scripting; see GIMP/Scripts

Getting Gimp

Windows

• Download Gimp for Windows.
• Download GIMP User Manual from same page (optional).
• Run the GIMP installer and User Manual installer.

Linux/FreeBSD/OSX, etc.

Your distribution should have pre-compiled packages available. Use the appropriate package management tool to install.

Documentation

• Gimp User Manual Has links to other resources.
• Short Tutorials
• Grokking the GIMP. This is Unix-centric and documents an old version but is still useful.