Backbone-associations 
Associations allows Backbone applications to model 1:1 & 1:N associations between application models and Collections. More importantly, applications can listen to any kind of change (change, add, remove, reset, sort, destroy) in this hierarchy using standard Backbone events and respond to them. (views can re-render for example). The implementation strives to be tiny (3.6KB), easy-to-understand, light-weight and fast.
For features, performance #s, API documentation, tutorials and recipes, please visit :
http://dhruvaray.github.io/backbone-associations/.
Who uses Backbone-Associations?
See https://github.com/dhruvaray/backbone-associations/issues/55
We encourage you to add your experiences to this list!