With all the electronic gizmos we have at disposal we must never forget that our brain never has dead batteries or is left at home. It pays to store these rules on the old internal hard drive... our brain.
The sunny 16 rule
The basic exposure for a scene taken in bright, sunny light is f16 and a shutter speed equal to 1/ISO setting. For example f16@ 1/100th at ISO 100.
The moony rule
My favorite formula is: f11@ 1/ISO setting for a full moon, same shutter speed at f8 for a half moon and the same shutter speed at f5.6 for a quarter moon.
Camera shake rule
Never shoot hand-held at a shutter speed slower than 1/focal length or 1/100 for a 100mm etc... The exception are the IS/VR lenses.
Always meter from a gray card rule
Metering a gray card is a excellent way to get a good mid-tone exposure. What do you do if you don't have one? Expose the palm of your hand and open up one stop.
When shooting for maximum DOF focus, shoot about 1/3 into the photo, This is because the DOF is twice as deep behind this point as in front.
The advice "expose for the highlights and let the shadows take care of themselves" is fine for slide film and digital but for print film you are better off overexposing one stop.
Flash fill rule
If your flash does not provide flash fill ratios you can manually do it. Set the flash ISO to twice the camera's ISO. Meter the scene, select the f-stop and set the flash to the same. This will result in a 2:1 ratio.
Flash range rule
How far will my flash reach? Double the distance, four times the speed. If a flash is good for 20 feet @ ISO 100 it will be good for 40 feet @ ISO 400.
Meter the area ABOVE the sun without including the sun. Set 1 F-Stop down to make the sunset look later or set the exposure compensation to -1.
The always rule
Always have your camera with you and be ready... for you just never know what will appear in front of your eyes.