gcolor2 is a basic colourpicker utility that allows you to select any colour on your screen, and, well, grab it's colour. It also allows you to save the colour you create to a file, which shows up as a list in the application.Discontinued
It was last updated in 2013.
Colorpicker is efficient through its interface. Every features are accessible from the toolbar and shortcuts. You have the possibility to choose any color thanks to the three RGB sliders (soon HSL, CMYK, ...)!
Rickrack is a free color editor. It is designed for generating a set of harmonious colors from color wheel or elsewhere. You can share these colors with your friends, or apply them into your creative works.
Open-Source Color Picker for Websites and Desktop Apps. It is an eyedropper tool to select color values from websites and desktop. Get a color reading from any point in your browser and from any point in your desktop.
ColorFish Color Picker vs Instant Eyedropper opinions
open-source and Linux support
This application is geared towards advanced users (developers, designers, etc…), who not only need to pick a color but also modify or view it in different formats. For simply picking a color on Gnome, the color-picker extension is far better suited.