Paint.NET (which is my favorite photo editor).
There are a few specialized tools, however, that need some introduction. The "Magic Wand" tool (fourth row on the left in the picture here) allows you to "magically" select the edges of a shape in the picture you're editing. This works well for shapes with defined edges against a solid-color background, but you can essentially click anywhere on the edge you want to to define and the magic wand will select the whole edge automatically. Remove the shape from the picture by pressing "Ctrl+X" and you can paste the shape into another image or fill in the area where the shape was in the original image.
The "Eye Dropper" or color select tool (five up from the bottom on the right) allows you to get the exact color of any pixel on the screen. Select the tool then click on a spot on the screen. That precise color will now be selected in the color wheel in your photo editing software. So if you wanted to fill in a matching color you could then select the fill tool and know that the color would be just the same as the object you selected. This won't work as well for complex shading where there are several subtle varieties of the same color with different saturation and hue (more on that later). But for brightly lit images with solid colors, the eye dropper does wonders.
Finally the "Clone Stamp" tool is a favorite of photo editors (fourth from the bottom on the left), but often misunderstood or not used at all by amateurs. What it does it sets two points on the screen of equal size the you select the distance apart for them. When you click the exact image under the first region is cloned and stamped on the second region defined by the tool. This is a very cool tool if you want to remove a blemish on a face. Just clone some of the normal skin over the blemished skin and the blemish disappears. Typically you need to zoom in pretty far when you're doing this (using the zoom tool that looks like a magnifying glass) so that the effect won't be obvious when looking at the picture zoomed out.
What tools do you find indispensable when editing pictures?