Free icon sets
An icon set is a family of icons drawn by the same designers, with the same grid, line width and corner style, so its icons look right next to each other. For a clean interface, pick one set and stay with it: Lucide and Tabler suit light, outlined designs, Phosphor and Material Symbols offer several weights of every icon, and filled sets work well for bold buttons and small sizes.
Every set is open-licence. The licence of each set is shown on its page and on every icon.