Copy Cat allows you to set up a checkbox that, when clicked, will copy the values from one field to another. This is particularly useful for situations like specifying a shipping and billing address. There is a good chance that the user wants to enter the same address for both fields. With Copy Cat, you can set a checkbox to copy user-entered values from the billing address field to the shipping address field.
This also supports simply copying the value from one field to another after the user has entered a value. For example, the user could enter her first and last name in a field and you could automatically fill this in the Name input in the Credit Card field.