Mixxx
The open-source DJ application: two or four virtual decks, a mixer, effects and support for both controllers and timecode vinyl.
Tue Haste Andersen started it in 2001 as a research project in Copenhagen into how DJs actually work, and it has since grown into a Qt application for Linux, Windows and macOS under the GPL. The feature list is what you'd expect from commercial rivals such as Traktor and Serato: BPM analysis and sync, key detection, loops, cue points and time-stretching via Rubber Band.
The big difference is controllers. Commercial packages mainly support their partners' hardware, while Mixxx maps MIDI and HID controllers with JavaScript that anyone can write. The result is a long list of community-built mappings for controllers their own manufacturers stopped caring about years ago.