It supports pressure sensitive graphic tablets and comes with an easy to use brush collection. There is a complex interface for creating your own brushes, focusing on brush dynamics (changes with speed, pressure, or randomly). The canvas size is unlimited and undo is supported, but not layers.
For adding little touches to your images IrfanView has a basic paint function built-in view it
In Windows 7 some of your old favorites now have a classy new interface view it
The full 10 part series on IrfanView view it
HyperSnap DX is a powerful and easy screen capture utility view it
You can use Note Everything to make all sorts of notes on your Android phone! view it