Interesting Features
Tablet Support
Acorn supports pressure sensitive tablets such as the Wacom Intous and Cintiq tablets. With the brush tool selected, you can press harder to get a thicker line or you can let up on the pressure to get a thinner line.
Smooth Strokes
There's a bit of fancy math inside of Acorn to convert roughly drawn lines into smooth strokes.
Quick Canvas and Image Resizing
If you hold down the control key when resizing a window, the canvas will grow or shrink accordingly. If you hold down the option key when resizing a window, the canvas will scale.
Next: Working with iPhoto