Expositor and it's free sibling Expositor Lite are handy slide-rule based exposure calculators used for manual photography. Pick the situation you're shooting, then slide the ruler until your camera's ISO is lined up next to it, and every combination of F-stop and shutter speed on the other side will get you close to a perfect exposure.

Use it in the field if you don't have a light meter handy, or use it to train your brain to be its own exposure calculator. Expositor is here to help you shoot great pictures!