Some of my Craft plugins

Twig Perversion is twig extensions that push twig in the direction of a real programing language. My favorite is the {% return %} tag, which lets a twig macro return something other than Twig\Markup

Picture makes it simple to do responsive images. {{ craft.picture.url(asset, 'thumb') }} will generate the html for a responsive image tag. All the complexity of srcset, sizes, and whether to use the picture or img tag is safely contained in one place in the picture config file. Picture grew out of my Responsive Images with Twig Macros article.

Snitch is half javascript. It inserts itself into backend pages, where the javascript looks for entry edit forms, and warns if another user is editing the same entry from another browser. Snitch relies on the structure of the html, which makes it fragile, subject to breaking in Craft updates. Still, it works well and is useful.