Ractive.js is a template-driven UI library that transforms your templates, styles and logic into blueprints for highly interactive apps. Ractive.js works for you, not the other way around. You do not need to learn new terminologies, concepts, and methodologies.
Sails.js make it easy to build custom, enterprise-grade Node.js apps. It is designed to mimic the MVC pattern of frameworks like Ruby on Rails, but with support for the requirements of modern apps: data-driven APIs with scalable, service-oriented architecture.
Intuitive and Documented It is strongly influenced by jquery with many similar methods so you'll feel at ease developing with Umbrella. However there are also some improvements such as with .ajax(), .append(), etc.
Both Knockout.js and Backbone.js have their strengths and weaknesses, but together they are amazing! With Knockback.js, you can use the strong ORM provided by Backbone and create dynamic views using Knockout bindings.