MapStuff is a project mainly written in C#, it's free.
Drawing on MKMapView (iOS) using MonoTouch
MonoTouch ports of two draw-geometry-on-MKMapView examples
http://spitzkoff.com/craig/?p=108
http://www.gisnotes.com/wordpress/2009/10/iphone-devnote-14-drawing-a-point-line-polygon-on-top-of-mkmapview/