How can I copy the hex color code of whatever pixel my mouse is hovering over?

Looks like this should do what you need.

• Instant Eyedropper
• Lifehacker article about it

Simply pick up the app's icon from your system tray, drag it toward the color you want to find, and a magnified, pixel-by-pixel view lets you pinpoint exactly what shade you're looking for. The color code—HTML, RGB, or whatever you prefer to set in the options—is instantly copied to your clipboard for easy pasting in any editing tool.

If you're using Firefox and would like to grab the hex color code within the browser, go to Tools > Developer > Eyedropper.

I, personally, like Color Cop.

Color Cop is a multi-purpose color picker for web designers and programmers. It identifies and represents colors in various hex color code formats. Simply drag the eyedropper control to any location on the screen and release. The hex color code for the selected color will be automatically copied to the clipboard.