What are some GUI open source tools that are the de facto industry standard (or at least a major player) in certain fields?

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The original was posted on /r/opensource by /u/real_kerim on 2025-05-03 22:55:37+00:00.

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I was looking at some open source GUI applications and was wondering about what niche open source software, if any, is out there dominating in a sector.

Something like OBS or Grafana. Or even Octave, which is basically the major competitor to MATLAB and becoming more popular in academia.