CascadingSelectsWithBackboneJS is a project mainly written in JavaScript, it's free.
Source code for http://blog.shinetech.com/2011/07/25/cascading-select-boxes-with-backbone-js/
ASP.NET MVC3 version by Cymen Vig Original Ruby on Rails version: https://github.com/shinetech/CascadingSelectsWithBackboneJS