Sample: Avalon DawsonDraw
Ryan Dawson posts a nice sample showing the basics of a drawing program in Avalon, including a floating toolbar and the ability to draw and select lines, rectangles and circles. He also provides the source.
BTW, Ryan’s pretty darn prolific on Longhorn and I recommend his blog in general. Also, if I remember correctly, he’s 19, so you’re getting pure enthusiasm and uncensored truth. Recommended.