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:

Instant Eyedropper is a free software tool for webmasters that will identify and automatically paste to the clipboard the HTML color code of any pixel on the screen with just a single mouse click.

From Instant Eyedropper

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:

The Ultimate CSS Gradient Editor was created by Alex Sirota (iosart). If you like this tool, check out ColorZilla for more advanced tools such as eyedroppers, color pickers, palette editors and website analyzers.

From Ultimate CSS Gradient Generator

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.