jQuery Foggle is a plugin that lets you interact with various form elements based on user-input. It lets you choose which elements to enable (or disable) while the user fills out the form.
While you can achieve the same functionality by writing some amount of JavaScript for each such field, this plug-in makes it a breeze. It takes only two parameters and does the same for you in just one line of code. 
Check out a demo at http://abhisek.iblogger.org/labs/foggle/