Personally I find the following plugins interesting: - Alpha operations (frei0r.alpha0ps): Display and manipulation of the alpha channel - Alpha shapes (frei0r.alphaspot): Draws simple shapes into the alpha channel - Color Distance (frei0r.colordistance): Calculates the distance between the selected color and the current pixel and uses that value as new pixel value - Defish (frei0r.defish0r): Non rectilinear lens mappings - Face detect (frei0r.facedetect): Detect faces and draw shapes on them using OpenCV - Lens Correction (frei0r.lenscorrection): Allows compensation of lens distortion - Luminance (frei0r.luminance): Creates a luminance map of the image - Rectangular Alpha mask (frei0r.mask0mate): Creates an square alpha-channel mask - Medians (frei0r.medians): Implements several median-type filters - Primaries (frei0r.primaries): Reduce image to primary colors - Color Selection (frei0r.select0r): Color based alpha selection - Tint (frei0r.tint0r): Maps source image luminance between two colors specified