JACK is a professional sound server daemon that provides real-time low latency connections for both audio and MIDI data between applications that implement its API. The API can be used through either JACK1 or JACK2.
Carla is an audio plugin host, with support for many audio drivers and plugin formats. It has some nice features like automation of parameters via MIDI CC (and send output back as MIDI too) and full OSC control.
BlackHole is a modern macOS virtual audio driver that allows applications to pass audio to other applications with zero additional latency.
VB-Audio CABLE is a Windows Audio Driver working as a Virtual Audio Cable. After installing the VB-CABLE driver, a new playback device appears in the audio device list and can be set as default device. VB-CABLE can be set as the default device, just like any regular audio device.
Soundflower is a Mac OS X system extension that allows applications to pass audio to other applications. Soundflower is easy to use, it simply presents itself as an audio device, allowing any audio application to send and receive audio with no other support needed.Discontinued
The project seems no longer developed. Last version, 2.0b2, released in December 2014, can be still downloaded from GitHub.
Create virtual audio devices to take the sound from applications and audio input devices, then send it to audio processing applications. Loopback is a software solution that provides the power of a high-end studio mixing board.
PulseAudio is a sound system for POSIX OSes, meaning that it is a proxy for your sound applications. It allows you to do advanced operations on your sound data as it passes between your application and your hardware.
SoundDesk is the easiest way to turn your macOS into a professional mixing desk.