I use Color Schemer’s ColorPix all the time match up colors. Say I want the Twitter feed box to match the sidebar color on my blog.

Open up ColorPix then open my blog; just move my mouse over the screen the color values on the left and the color swatch on the right will change color depending on the pixel under my mouse. Hit the spacebar, and the color locks. Then I can copy the hex code color, cut and paste the color code wherever needed. Color matching and capturing made easy.