ColorPix is a fast, free Windows color picker
After trying a number of different applications, I was delighted to find ColorPix (an app from ColorSchemer, who is the developer behind the previously mentioned ColorSchemer Studio 2).
ColorPix has an interesting lock and unlock mechanic for selecting the color that you want to find. While the application is active, hitting any key will unlock the picker, which you can then move anywhere on your screen. Once you've found the color, hitting any key again will lock it in so that you don't accidentally nudge the mouse while clicking to make your selection. It also includes a magnifier window to help with pixel precession at several different levels of zoom.
Once you've locked in a color, you can click anywhere on the RGB, HEX, HSB, or CMYK values and copy the individual color for the mode that you're working in directly to the clipboard. If you want the HEX value, you can use the Ctrl+C shortcut to get it even faster.
My typical workflow consists of hitting Alt+Tab (to make ColorPix active), hitting Space to unlock the picker, finding the color I want, hitting Space again to lock it in, using the Ctrl+C shortcut to copy the HEX value, and then pressing Alt+Tab again to go back to the application that I'm working in. The grand total of time that I'm using is usually less then 15 seconds.
The only two features that I'd love to see added would be support for the HSL color mode and an option to change the RGB scale from 0-255 to 0-1.
ColorPix weighs in at about 600 KB and doesn't require an installation; that makes it perfect for storing on Dropbox so that it is readily available whenever I need it.
Apps like ColorPix make my workflow a little faster and my life a little easier. It's become one of my favorite productivity boosters that I use any time I'm doing design work. If you do a lot with color, it's definitely worth a download.