Pencil Tool
The pencil can be used for drawing 1-pixel at a time, or what is essentially a non-antialiased freeform brush. This is similar, but not the same, as using the Paintbrush with a brush setting of 1 and without antialiasing.
Pencil Tool
Tip
It can be useful to use the pencil tool with the grid mode enabled while you are zoomed in on an image to do per-pixel editing. In the following example, the pencil tool was used to draw the bottom and right edges of an icon:
Example
Original Image
After using the Pencil Tool to complete the image