Mac: Frank is a handy little app that can grab any colour from any pixel on your screen upon request. Just call it up via its global, user-specified hotkey, select the coloured item of interest, and it'll provide you with a variety of useful colour codes.
If that isn't handy enough, the menubar app also works beautifully with Photoshop (CS5 and above). You can establish a remote connection so that any colour Frank picks up is set as Photoshop's foreground colour. Whether you're designing or developing, it's a great timesaver. To see how Frank works, check out the video above.
Frank DeLoupe ($1) [Mac App Store]