System UIcons is a collection of open source icons designed for products and systems in mind. Each icon is on a 21x21 grid.
Use the icons how you want, for free, and without any attribution.
https://systemuicons.com/
Martin Zlámal has created an awesome open source community icon group over at System UIcons React