JMicroVision - Image analysis toolbox: measuring and quantifying components of high-definition images
RealWorld Paint - RealWorld Paint is an free portable image/photo/animation editor.
ImageJ - and Other Health Care
JugiPaint - JugiPaint is a painting software with diverse tools for various types of illustration work.
Fiji - Fiji: A batteries-included distribution of ImageJ.
Paint +++ - Paint +++ is a native image editor for Mac that enables you to complete everyday image editing tasks in a single place.