ColorGrab

A cross-platform color picker, with a simple space-efficient user interface similar to Color Cop.
Select any color from screen, and convert between RGB, HSL, HSV, and CMYK.

Can magnify the screen (and the magnified area can be copied to the clipboard) and remembers the settings and positions of windows when they are closed.
Allows dragging and dropping colors to/from other programs, and displays color representations for use with HTML, CSS, etc.
Also has support for saving, loading, and managing palettes (using GIMP's palette format).