How to create custom control
http://www.silverlightshow.net/items/Creating-a-Silverlight-Custom-Control-The-Basics.aspx
Zoom control
http://zoomcontrol.codeplex.com/releases/view/35970
Gauge
http://www.codeproject.com/Articles/38361/Circular-gauge-custom-control-for-Silverlight-3-an