DoF Viz is a depth of field calculator that works in a completely visual way, allowing you to see the in-focus region in relation to a typical subject - a human being.
Two modes are provided - a quick mode which allows you to pick common settings using sliders, and a precise mode allowing full control.
Additionally, the Distance Estimator tool makes it easy to manually gauge the distance between you and your subject.
Configuration options are available for full-frame, APS-C and Micro Four Thirds sensors.
Free for 30 days - upgrade for $2.99 (price in other regions varies).