Voicemeeter is Audio Mixer Application endowed with Virtual Audio Device used as Virtual I/O to mix and manage any audio sources from or to any audio devices or applications.
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.