SystemJS is described as '* Loads any module format when running the ~15KB development build' and is an app in the Development category. There are three alternatives to SystemJS for Linux, Mac, Windows, BSD and Node.JS. The best alternative is Webpack, which is both free and Open Source. Other great apps like SystemJS are Brunch (Free, Open Source) and Browserify (Free, Open Source).
Brunch is an assembler for HTML5 applications. It is a simple but powerful build process and pipeline. It’s agnostic to frameworks, libraries, programming, stylesheet & templating languages and backend technology.
Browsers don't have the require method defined, but Node.js does. With Browserify you can write code that uses require in the same way that you would use it in Node. Browserify is a tool for compiling node-flavored commonjs modules for the browser.