Ardour
The DAW that proved studio-grade multitrack recording could be done as free software on Linux.
Paul Davis started writing Ardour around 2000, and version 1.0 arrived in 2005. To move audio between programs with low latency he also wrote JACK, which became the backbone of Linux audio for nearly two decades. Ardour is licensed under the GPL, runs on Linux, macOS and Windows, and loads LV2, LADSPA and VST plugins.
The business model is unusually honest: the source is free, but anyone wanting ready-made binaries is asked to pay what they think is fair. Harrison Consoles builds its commercial Mixbus on top of Ardour, making it a rare case of a mixing-desk maker selling a GPL program with its own badge on it.