When you create a form for your user to fill up credit card number, phone number, serial number or ect. and you want to make your form automatic jump to the next field when your user finished filling it.
Autotab is a jQuery plugin that provides auto-tabbing and filtering on text fields in a form. Once the maximum number of characters has been reached within a defined text fields, the focus is automatically set to the defined target of the element.
This script I found on lousyllama.com, it make your form easily to fill up, powerful and you can download free.