I'd like to second the architecture side of this question. I'd love to see more on why and where a particular technology fits into web apps. Recently there has

I'd like to second the architecture side of this question. I'd love to see more on why and where a particular technology fits into web apps. Recently there has been a lot of popularity with Node.js, Backbone.js, and even Knockout.js. How and Where do these fit? What design decisions were made to achieve this?