Kimai

Kimai is a free, open-source, self-hosted time tracking application. It supports multiple users, projects, and activities, with features for exporting data, creating invoices via plugins, and tracking expenses. It is highly customizable and extensible. It targets freelancers, small businesses, and developers who need a free, customizable tracking tool they can host on their own server.

It scores decently for its complete freedom, zero cost, and active community. Its ranking reflects the technical barrier to setup and maintenance, but for those able to self-host, it provides a powerful, private alternative to commercial services.