ColorPix is nice because it is small in all ways, on the screen (so it doesn't get in the way when you are sampling a colour) and the program itself is just one exe file of 605KB! You don't have to install it so you even could keep a copy on your USB flash drive.
It's easy to use too, just put the mouse pointer over something and the colour is shown in ColorPix - press a key and a lock symbol shows the colour is locked into ColorPix so you can take note of the RGB, hex, etc, numbers for that colour. The magnifier feature is very useful, where ever you put the pointer will be enlarged, you can change the zoom to whatever you want to see the most detail - see the example screen shot above.
Find out more and download it from here:
Sometimes the simplest applications are the best, this is a classic example!