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.