Are there any hidden gems that a newbie might not know about?

For example, it took me half a year of MacOS usage before I discovered rectangle.app [1] which allowed you to position screens by pressing a key combination.

It took me 9 months before I happen to run into Stats.app [2] which gives you the option to put certain stats (like GPU temperature, CPU load, or RAM load) into the top bar on MacOS.

What else am I missing? What brilliant app is waiting out there for me to find her?

[1] http://rectangleapp.com

[2] https://github.com/exelban/stats