dhtmlxColorPicker allows choosing a color by modifying HSL or RGB values. You also see actual HSL/RGB indexes while choosing a color in the palette.
dhtmlxColorPicker allows the user to add colors to a custom colors palette. Chosen colors are saved in cookies, so the user will see them next time he returns to the page.
dhtmlxColorPicker can easily be attached to an existing input control (by adding one attribute to the input tag). The control then gets a small square box with the chosen color as background.