miniPaint - Online graphics editing tool use HTML5 technologies like Canvas, Offline support, Drag and Drop.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
RealWorld Paint - RealWorld Paint is an free portable image/photo/animation editor.
Invision - Prototyping and collaboration for design teams
Paint XP for Windows 7 - Standalone old MS Paint program that allows you to get your old XP Paint interface in Windows 7.
Sketch - Professional digital design for Mac.