QOwnNotes
A simple file-based markdown editor with rich features. Built on Qt (C++), it's the fastest note-taking application I've ever used. It's quick, light, and supports encryption and cloud integration.
Handbrake
The only non-bs video converter in the whole damn universe. It's free, opensource, and available for all platforms. Don't even think about other stuff! Go! Download it! Keep it!
MP3tag
Take a look at your music library. Seriously? What's that abomination? Use this and fix your album arts and other ID3 tags! It looks pretty on smartphones.