Fontain collects free, libre, and open-source fonts and, unusually, includes the source files alongside the compiled font, not merely the finished .ttf or .otf.
Having source files matters if you actually want to modify a typeface - most free-font sites only give you the finished product.
Aimed more at developers and type-curious designers than someone who just wants to download and use a font as-is.
Fonts here (Roboto, Fira Code, Source Code Pro among them) are generally under OFL, Apache, or similar permissive licenses that allow commercial use, but the exact license affects what you're allowed to do with modifications - check which one applies before redistributing anything you change.
A selection of free/libre/open source fonts together with their source files.
