Daily

• flameshot for screenshot with annotations
• quodlibet for playing music (nice GUI, scriptable with python - wrote a few myself)

Occasionally

• czkawa for deduplication
• manuskript for writing
• tuxguitar for guitar tabs

Dunno how known they are, but it took me a while to find them.

CC BY-NC-SA 4.0

Space sniffer http://www.uderzo.it/main_products/space_sniffer/

It was on life hack.com a decade ago. Literally haven’t seen a single app beat this one.