The Ctrl.tool allows you to choose a point from the canvas and make the color of that point the active foreground color. When a paint or drawing tool is selected the dropper tool can be quickly accessed by pressing
There are several options for the dropper tool in the Tool Options dock. The first drop down box allows you to select whether you want to sample all visible layers or only the active layer. You can choose to have your selection update the current foreground color. You can choose to add any colors selected by the dropper into a color palette. One other option is to set a sample radius, which will average the colors in the area around the cursor.