Paint-App is a project mainly written in Objective-C, it's free.
Custom UIView subclass to enable a drawing overlay in iOS
This is an app that tracks touches in a custom subclass of UIView (called CanvasView) and draws them by implementing the - drawrect method. This view is positioned on top of a sample background image and is transparent.
Paths are added to an NSMutableArray to allow functionality for undo. It is currently limited to Line objects, although other shapes are perfectly able to be implemented.
Future Plans:
Feel free to expand the functionality to meet your needs. This is a very simple implementation that was done as an exercise.