We have thread for terminal applications, but not GUI ones. We should fix this.
I'll try posting lesser known applications, so no Krita or Blender here.
Flameshot is tool for taking screencaps of your screens, but it has few editing options that make showing people what you want to show so much nicer. Once you capture your screen, you can crop it, draw on it, blur some parts, … all before saving. And it's so much quicker than doing it after save with another application that take long to load and then export.
LBRY – this is kinda popular lately, but it deserves notion for being really cool youtube alternative. Try it.
AntiMicro – map your gamepad to keyboard. Good for games that don't work with your gamepad for some reason or controlling your movie player / browser with something like Wii remote or any other gamepad.