I often have to select colors from images for use in my HTML. Taking a screenshot to use image editing tools to capture that color is inefficient, and the images I am sourcing aren't always coming from a web browser to be able to use browser extensions for that purpose. That's why I like a desktop tool:
This is a pretty handy tool for Windows that quickly captures colors from anywhere on the screen, and is significantly faster than other color pickers I've used in the past.
I was trying to put together a nice background gradient in CSS that uses alpha values in the color to indicate transparency. There are a handful of tools out there that do it, but The Ultimate CSS Gradient Editor has everything I needed:
Color pickers, sliders for positioning colors within the gradient, presets, a preview and a simple copy to paste output. All that, and it supports transparent colors in the gradient. Very nice.