Embed interactive code view
For software engineers and data scientists. Great or JavaScript - supports NPM packages and changing Node versions. - https://runkit.com/runkit/d3-example-from-beaker
Add audio clips to your website. Nicely-designed ones. Not that ugly little HTML player.
elfsight
https://senja.io/testimonial-widgets (I like their hero website design)
Not quite a widget, or a feature for that matter. Very funny scroll effects…