Air Draw is a drawing program with a strong sense of object center and symmetry. It also features per layer tool selection, a nice pallet, and the ability to automatically decrease the alpha for inactive layers.
Note: it requires Adobe Air.
It’s pretty stable although the interface is a bit rough still. The “more” box has toggles for symmetry and a draw/erase toggle, the color picker has red green blue hue saturation value and alpha sliders, and the sliders next to the layers are alpha-while-active and alpha-while-inactive. The pointer tool is a color picker. Z is a hotkey for clear layer and most things are undoable (clear layer, resize canvas, draw, erase, delete layer). It can save as a custom file format with layer information or flatten and save to a png.
Here are a couple of sketches made in a recent version.