Colorcombinate is a color palette generator and viewer, that helps to compute color harmonies. It includes a GTK widget library implementing a GTK dialog, to include such functionality in your own applications. It comes with python bindings as well.
This tool is indispensable! The UI is simple and intuitive and it includes hex codes, HSV, and RGB. I keep this alongside Beyond Compare, Regex Buddy, and Balsamiq as part of my must-have development toolbox. I can't work without it.