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)

FFTW -- Fast Discrete Fourier Transform. Reason: it routinely beats proprietary FFT routines on real & complex and single & multidimensional transforms, and with better accuracy.

Video downloading: youtube-dl

https://github.com/rg3/youtube-dl

That may sound silly but humble Notepad++ should be a part of your list of great opensource software. Although at first glance it does not require spending time on learning its functionalities it offers a lot of plugins that allow to replace proprietary software (i.e. I'm using XML-plugin and that allows me to work with XMLs without using specialised software like Altova).

Also TrueCrypt - I'm using it all the time (although there are changes in maintenance community the last stable version was proven to be clean - I still trust old good truecrypt)

Radio Station Programming: LiquidSoap http://www.liquidsoap.fm/

Streaming: Icecast http://icecast.org/

Low bandwidth, low latency audio codec: Opus http://opus-codec.org/

Lossless audio codec: FLAC http://xiph.org/flac/