Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.
Hijack node require calls, with support for hijacking the entire module itself rather than only its properties. Handy for unit testing and mocking require calls in sub modules!