VLC Media Player - VLC is a free and open source cross-platform multimedia player and framework.
VCV Rack - A cross-platform modular synthesizer.
foobar2000 - An advanced freeware audio player for the Windows platform.
miRack - Eurorack-style modular synthesizer for iOS.
Winamp - Winamp is a media player that allows users to play multiple-file formats, arrange media in a varied file-management system, and play unique media with AAC encoding.
SynthEdit - Visual programming software to build your own synthesizer.