Since it was asked in one of the SAR threads I figured I would post up some images of the more common types of helicopters seen around the valley area and on the trails. This is by no means an exhaustive list but should help you identify military, police and rescue helicopters.
Mesa PD - MD500
Phoenix PD - Agusta 119 Koala
MH-53 Pave Low (another variant is the CH-53)
The following helicopters are not commonly seen around the valley but may be seen down towards Yuma flying out MAS Yuma (Marine Corps Air Station):
CH-64E Sea Knight
As you can see the 46 and 47 are basically the same aircraft.
If anyone has any comments, corrections or additions please feel free to post up and I will add to the list as they come in.