Periodically Updated List of Free and Open-source Music Tools

Audio Analysis Applications (for analyzing audio in various ways)

EAnalysis (sound-based music analysis application)
Orban Loudness Meter (real-time and offline loudness unit meter and analyzer)
Sonic Visualiser (application for viewing and analyzing the contents of music audio files)
SPEAR (FFT-based audio analyzer with editing and resynthesis capabilities)

Audio Editors (for recording and editing audio)

Audacity (recording and editing software with effects like Paul stretch and noise reduction)
oceanaudio (fast and functional audio editor)
WavePad (full-featured audio and music editor)

Audio Routing Tools (for routing audio between applications)

JACK (audio connection kit for applications to communicate with each other)
Soundflower (MacOS system extension that allows applications to pass audio to other applications)
QjackCtl (simple Qt application for controlling the JACK sound server daemon)

Digital Signal Processing Tools (for engineering, prototyping, and simulating audio signals)

FreeMat (environment for rapid engineering, scientific prototyping and data processing)
GNU Octave (mathematics-oriented syntax with built-in plotting and visualization tools)
LiveSPICE (SPICE-like circuit simulation tool for live audio signals)
LTspice (high performance SPICE simulator, schematic capture and waveform viewer)
MacSpice 3 (electronic circuit simulator and analyzer for OSX)

DJ Software (for mixing tracks, beat matching, and applying audio effects)

Mixxx (DJ application)

Media Players (for playing audio and video files)

VLC (multimedia player)

Notation Software (for creating scores)

Denemo (LilyPond-based music notation program for rapid notation entry)
Frescobaldi (LilyPond-based sheet music text editor)
LilyPond (music engraving program)
MuseScore (music notation software)

Plug-in Effects/Utilities (for processing sound in host applications)

Blue Cat FreqAnalyst (free spectrum analyzer plug-in)
Voxengo (suite of audio effect plugins)
VST 4 FREE (database of free VST instruments, effects, and MIDI plugins)
KVR (database of plugins and apps, with option to filter for free ones)

Plug-in Hosts (for running plugins without a Digital Audio Workstation)

Pedalboard2 (VST and AudioUnits host with MIDI and OSC control, designed for live use)
VSTHost (free VST host for Windows)

Plug-in Builders (for designing your own plugins)

SonicBirth (AudioUnit designing application)

Programming Environments (for creating instruments, sounds, compositions, interactive systems, etc.)

ChucK (strongly-timed, concurrent, and on-the-fly music programming language)
Csound (sound and music computing system)
Gibber (creative coding environment for audiovisual performance and composition)
Nyquist (sound synthesis and composition language)
Pure Data (visual programming language for multimedia)
SuperCollider (platform for audio synthesis and algorithmic composition)

Rhythm Tools (for creating and playing rhythmic patterns)

Braid (Python module for livecoding polyrhythms)
Euclidean Pattern Generator (Java application for creating Euclidean rhythms)
Hydrogen (advanced drum machine and pattern editor)

Sequencers/Digital Audio Workstations (for composing on a timeline with audio and MIDI)

Aria Maestosa (open-source midi sequencer/editor)
Helio (linear-based sequencer with a clean interface)
LMMS (sound generation system, synthesizer, beat/baseline editor and MIDI control system)
LNX_Studio (digital audio workstation created in the SuperCollider language)
MusE (MIDI/Audio sequencer with recording and editing capabilities)
Reaper (digital audio production application for Windows and OS X)
SunVox (small, fast and powerful modular synthesizer with pattern-based sequencer)

Software Synthesizers (for generating digital audio)

Dexed (plugin synth closely modeled on the Yamaha DX7, also serves as a midi cartridge manager for the DX7)
Synth1 (full-featured 64-bit VST synth)
Bfxr (computer game sound effects generator)

Sound Processing Applications (for processing sound in various ways)

Cecilia (audio signal processing environment)
Guitarix (virtual guitar amplifier)
Soundgrain (graphical interface for controlling granular sound synthesis)
Soundhack (spectral sound processing application)
Spatium (modular software tools for sound spatialization)

Sound Libraries (for finding sound samples)

Internet Archive (millions of audio samples, videos, books, etc.)
British Library Sounds (sound and video recordings from all over the world)
Freesound (collaborative database of sounds)
Philharmonia Orchestra Sound Samples (orchestral sound samples)

Last update: 5/4/2017