Just Color Picker - Free portable colour picker and colour editor for web designers, photographers, graphic designers and digital artists. Supports Windows and macOS.
Instant Eyedropper - Identifying the color code of an object on the screen is usually an involved, multistep process:...
gpick - A color picker and color scheme creation tool.
ColorZilla - ColorZilla is an extension for Mozilla Firefox and the Mozilla Suite.
gcolor2 - GTK+2 Color Selector
Sip - A better way to collect, organize & share your colors.