Simple Time Tracker let's you do exactly what you might think. Just track your time.
The app is quite simple and does not overwhelm you with a lot of settings and stuff. Just create a timesheet and click "start" - that's it!
If you want to you can enter a text note on what task you are working on.
You can create multiple sheets and switch between them. When you're done with working on a specific project you can export a CSV file with your tracked time spans (including start and end dates of tracked events as well as notes).
The UI is simple and just shows you the title of the selected timesheet, the current duration as well as the total time you spent working on it.
Use a second window to view every single entry of the selected sheet by clicking the "edit" button. By selecting multiple entries you can sum up specific tasks.