Desktop productivity tool No. 4: GIMP 2.6
The name is an acronym for GNU Image Manipulation Program, and among free software advocates GIMP has become a staple recommendation as an alternative to Photoshop. Many of Photoshop's tools are re-implemented here: layers, editable text objects, support for a remarkable number of image formats, even support for Photoshop's own brushes. One major plus is the panoply of third-party scripts and add-ons that extend the program's functionality. But the program's biggest minus remains, even after many years, its clunky user interface -- as shown in elements such as the non-native file picker dialog in the Windows edition.
GIMP implements many of Photoshop's functions, including the ability to use Photoshop's own brushes.
Desktop productivity tool No. 5: Paint.NET 3.5.5
This spectacular program comes closer than any other to replacing Photoshop for the casual user. It's neither cross-platform (it's Windows-only) nor open source (although it is free for use). But it's far more elegantly presented and immediately useful than its open source counterpart, GIMP. Layered editing, a gallery of plug-ins, tons of built-in effects (even some GPU-accelerated ones), and support for just about every major image format make this program well worth having. Paint.NET has some weaknesses -- namely, text handling is awkward, and there's no way to create nondestructive manipulation à la Photoshop's adjustment layers. Both of those are being worked on for future editions of the program, though.
Paint.NET's toolset is close enough to Photoshop that most people can start using it with minimal retraining.