I’m not aware of any that are recently mentioned - this used to be popular but I never hear anything about it now:
At one point I separated out the css,js and img handling from jekyll and handled it on my own via Gulp. You can also do something similar with NPM. It actually worked really well.
I am too afraid of node/NPM. I want all my jekyll sites to be low maintenance and it seems node is a whole new world I need to understand, so would like to avoid a dependency on it. Although I see that it is so widely used in all kinds of projects that I probably have to figure it out at some point…
Thanks for the link, I will check out what you did there.