Acorn

Key Features: Layer styles, number of filters, curves and levels, blending modes, brush designer
Supported File Formats: Native file format .acorn, JPG, PNG, GIF, PSD, SVG, RAW, PSD
Supported Platforms: Mac OS X
Price: Paid

Paint.NET
Key Features: supportive tools, layers, curves and brightness, simple photo editing
Supported File Formats: RAW, BMP, JPEG, GIF, PNG
Supported Platforms: Windows
Price: Free

Sumo Paint
Key Features: Layer Effects, Professional painting tools, Filters, Gradient tools, Selection and Shape tools, Image Enhancement, Clone Stamp tool
Supported File Formats: JPEG, PNG, GIF
Supported Platforms: Browser or iPad (Web Based)
Price: Free for basic, Paid for pro version

Seashore
Key Features: Layers, 6 Gradients, Brushes, Image Editing tools, Anti-aliased Brush
Supported File Formats: JPEG, PNG, TIFF, XCF, PDF
Supported Platforms: Mac OS X
Price: Free

Adobe Photoshop Express
Key Features: Simplicity, Optimized for touch, compact to be handled with fingertips
Supported File Formats: JPG, Raw, TIFF
Supported Platforms: Windows, iOS, Android
Price: Free