Many of the following programs can be used as an alternative to Adobe Photoshop (a popular raster graphics editor).
FireAlpaca, Windows and Mac OS X, free
PaintTool SAI, Windows, $50
Paint.NET, Windows, free
GIMP, free, Windows, Mac OS X, and Linux
This program is very powerful, but can be difficult to use.
Pixelmator, Mac OS X 10.9+ (App Store), $29.99
Pixlr Editor, web-based
This also has iOS and Android versions.
Affinity Photo, Windows and Mac OS X (10.7+), $50
Acorn, Mac OS X 10.10+ (App Store), $30
PaintShop Pro, Windows, $80
Seashore, Mac OS X, free
Artweaver Plus, Windows, $40 (a limited free version is also available)
Krita, Windows and Linux, free
Pinta, Windows, Mac OS X, and Linux, free
Xara Photo & Graphic Designer, Windows, $90
Photo-Brush, Windows, $55