What app is so useful, you can’t believe it’s free?

Quite a few of my favorites have already been mentioned, so I'll add some that live on my toolbar:

Zim, a desktop wiki with markdown and a lot of plugins. Great for organizing all of your notes with links and a fast search function.

Heroic launcher, for organizing your Epic, GOG and Prime collection.

Geany, an extremely configurable and light editor that can be as simple or as full featured as you want, via plugins.

Terminator, a solid multi terminal emulator where I spend most of my time at work.