Kate and Okular count as "aren't well known"? Because I love them.

Also I love the Global menu for Plasma.

KolourPaint is my pick. I love that there's a paint equivalent for quick jokes while screen sharing or trash drawings

Okteta. A hex editor is always useful.

qBittorrent, I use it instead of KTorrent

K3b for CD ripping, Kid3 for tagging audio files!

Qmmp and Kpat.

The Kasts podcast player is great

qView simple and fast image viewer, I don't need the bells and whistles of most image viewer apps, qView is a simple to the point app like old feh but integrates well will KDE.

I use Strawberry too and in general it is great. Sadly it still lacks some features I need, so I still need foobar2000 with wine on the side (for library management, stuff like viewing and editing all custom tags, renaming files according to tags instead of copying them, ...)

RKward, a great R IDE!

Firealpaca is how I discovered kde and I don't see it mentioned too often, so I will take my chances and say that. However it's the only kde program I know of that isn't FOSS. But I typically prefer it's simplicity over the bells and whistles of gimp and sometimes krita.