I'm not gonna mention the basics like Kate. They're great but nothing new.
My 2 hidden gems that I use on a daily basis are:
QOwnNotes for markdown note taking. Only competent desktop app I found that comes without any electron bullshit.
Nyrna to send a game to sleep when I want to take a break or get interrupted. Saves me from booting it up again when I want to pick up where I left off.