Sea.js
A Module Loader for the Web
Script.js
Asyncronous JavaScript loader and dependency manager
LABjs
Loading And Blocking JavaScript: On-demand parallel loader for JavaScript with execution order dependencies
Lazyload
Tiny, dependency-free async JavaScript and CSS loader.
RequireJS
A file and module loader for JavaScript
Head js
The only script in your HEAD. Head JS loads JavaScript files in parallel like images without blocking the page. Your page will be faster. Even with a single combined file.