Ask HN: Best open source/free software in their own category?

I recently stumbled upon Krita on HN, which I really like. Would be great to collect the most powerful, feature-rich free software that are among the best in their category, and that get close to or even beat the commercial ones. I know some other great pieces like Gimp, Inkscape, but what's the state of the art for animations, music notations, audio mixing, etc? GUI & CLI should be both ok.

Here is a list of my favorite software that are worth learning:

  - Gimp: photo editing
  - Krita: digital painting, illustration
  - Inkscape: vector drawing
  - sox: sound processing (command line)
  - ffmpeg: video processing (command line)
  - imagemagick: image processing (command line)