ColorSnapper - The missing color picker for Mac.
Just Color Picker - Free portable colour picker and colour editor for web designers, photographers, graphic designers and digital artists. Supports Windows and macOS.
Sip - A better way to collect, organize & share your colors.
ColorSlurp - Pick, edit, save, and copy colors. The best color picker in the universe!
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gcolor2 - GTK+2 Color Selector