Expositor is a handy slide-rule based exposure calculator for photography. Pick the scene you're shooting, then slide the roller until your camera's ISO is lined up beside it, and every combination of F-Stop and shutter speed on the right will get you close to a perfect exposure.
For a more creative example, let's calculate shooting sports on an overcast day with a shallow depth of field (F4) to isolate the subject, and a fast shutter speed (1/500s) to stop motion. Match the F-stop and speed, then check the ISO value next to 'Heavy overcast day' to see how sensitive your camera needs to be. In this case, it's ISO 200!
Use Expositor 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!
More instructions and detailed usage examples can be found at http://expositor.wunderkemmer.com/Instructions.html.