CanJS and Backbone both provide structure for JavaScript applications through an MVC-like pattern.
CanJS is a lightweight MVC framework built to provide structure for JavaScript applications. It is aimed at developers of any library who need a battle-hardened library that makes building complex applications easy. It is part of a larger offering named JavaScriptMVC from Bitovi (its creators) that has documentation, testing and dependency management tools.
Backbone is a flexible, minimalist solution for seperating concerns in a JavaScript application. It provides just a little bit of structure to a jQuery application and has largely attracted developers who know just enough jQuery to get by and now want to organize their code a bit more.