First of all here is the main link for Backbone.JS page on github
Tutorials:
Developing Backbone.js Applications
Relational:
https://github.com/PaulUithol/Backbone-relational - "Backbone-relational provides one-to-one, one-to-many and many-to-one relations between models for Backbone."
Validation:
https://github.com/thedersen/backbone.validation - "A validation plugin for Backbone.js inspired by Backbone.ModelBinding, and another implementation with a slightly different approach than mine at Backbone.Validations."
Frameworks:
http://derickbailey.github.com/backbone.marionette/ - "Backbone.Marionette is a composite application library for Backbone.js that aims to simplify the construction of large scale JavaScript applications. It is largely a collection of common design and implementation patterns found in the applications that I (Derick Bailey) have been building with Backbone, and includes various pieces inspired by composite application architectures, such as Microsoft's "Prism" framework."
Finally you can find more on:
github:
https://github.com/search?q=backbone&repo=&langOverride=&start_value=1&type=Repositories&language=JavaScript
bitbucket:
https://bitbucket.org/repo/all?name=backbone
google code:
http://code.google.com/hosting/search?q=label:Backbone.js