ActivityWatch
An app that automatically tracks how you spend time on your devices.
Kimai
Kimai is a free & open source timetracker. It tracks work time and prints out a summary of your activities on demand.
TimeTagger
An open source time-tracker based on an interactive timeline and powerful reporting.
Traggo
Traggo is a tag-based time tracking tool. In Traggo there are no tasks, only tagged time spans.