Nanobar
Very lightweight progress bars. No jQuery
Imagesloaded
JavaScript is all like "You images done yet or what?"
Lazy Load Xt
Lazy load XT is a jQuery plugin for images, videos and other media
RequireJS
A file and module loader for JavaScript
jQuery Pjax
pushState + ajax = pjax
Webpack
Packs CommonJs/AMD modules for the browser. Allows to split your codebase into multiple bundles, which can be loaded on demand. Support loaders to preprocess files, i.e. json, jade, coffee, css, less, ... and your custom stuff.