This website itself is fully open-source, ready to explore. I love open source! You can see a description of the technologies used to power this website there.
The source code for it is a bit more complicated than you'd expect, because it not only features a custom experimental build system, but it also doubles as a simple static Markdown-based blog generator I'm constantly iterating on, and it features a development server that makes updating content and layout pretty easy.
A lightweight, simple, and unopinionated front-end framework created by Leo Horie. I am an active core contributor, but most of my work circles around performance (where this truly shines), stability, and making it easier for people to contribute where they feel best. It's light enough and fast enough that it flies even on mobile, where there's little to no processing power.
A very simple, minimalistic, and flexible module loader that racks up a whopping 528 bytes minified and compressed. It supports many of the features you would expect of a larger, heavier module loader, including worker support and remote loading, but in a fraction of the size. It is fully tested, and it has a very simple API that should function on just about any browser you could potentially run it on, including old IE.
It does not provide any particular facilities for bundling, but you could just concatenate all your modules into a single script at the end and call it a day. And unlike Webpack and similar, loading bundles remotely is pretty easy to do, and hot swapping modules is even easier.
It was created as an experiment to see how small I could create a functional, usable, module loader that could actually potentially scale beyond a small toy project. I myself don't actively use this yet, but I've already had others tell me they were interested in it themselves.