If you're colourblind or just need to constantly know what colours are on your screen the free program Colorblind Assistant might be for you. The program tells you whatever colour is displayed at the current location of your mouse/trackpad pointer.
The program gives you both RGB and HTML colour values and also displays RGB, brightness and saturation values on a bar graph. You can toggle whether or not the program stays on top -- with the small footprint it doesn't really get in the way. Also there is a zoom pane for fine pixel detection.
The developer, Achronism Studios promises that the program is spyware and advertising free. There is a PayPal donation suggestion when you click on the About button, but besides that I couldn't find any issues with program other than it very rarely gives the wrong colour name such as "dark magenta" or "light grey" when on a colour that looks like purple to me. This flaw aside, the software performs as advertised and the price is certainly right.