👥 MeetingBar
MeetingBar is an indispensable utility for me, a person who has to attend meetings. You can configure MeetingBar to work exactly how you want; being able to see my next meeting at a glance in my menu bar is amazing, and the full screen notifications I've set up are impossible to ignore and ensure I'm always early, never late.
On top of all that, it's open source and completely free. There are some optional in-app purchases and other options to provide the developer with some financial compensation (I'm a member on their Patreon), but all functionality is available without paying a dime.
If you have any meetings on your calendar you should give it a try.