ZynAddSubFX is a opensource software synthesizer capable of making a countless number of instruments, from some common heared from expensive hardware to interesting sounds that youll boost to an amazing universe of sounds.

ZynAddSubFX Key Features:
1. Realtime synth
2. Polyphonic synth (allows playing more than one note at once)
3. Mutlitimbral synth (allows playing more than one instrument at once)
4. Microtonal capabilities with any scale, up to 128 notes per octave, and key mapping
5. Legato, Poly, and Mono modes
6. Instruments can be organized in kits, which allows you to make drum kits or layered(mixed) instruments; this makes possible to use more than one instrument for a single part. It is possible to choose what items from the kit should be processed by the Part's effects.
7. Filters up to 60dB/octave (or more..); filters can be analogic (LPF,HPF, BPF,etc.), formant(advanced formant filters) and state variable (SVF)
8. Envelopes can have ADSR (or ASR, etc..) modes or can be free modes (with any shape)
9. Many settings have a "randomness" option, which you can make instruments that every time produces a subtle different sound; this produces a very pleasant effect (like analogue synths or natural instruments that can't produces exactly same sound twice). Because of this, even if it is digital synthesis, it sounds "warm". Listen the ogg vorbis demos from this page to hear this effect.
10. The specifications can be easily changed, so (for example) if you want to change the number of voices or parts to a higher value or to allow a higher number of effects at the same time, there is necessary to change only a single value from the source file (globals.h) and to recompile the program
11. Intuitive and easy to use User Interface with Clipboard and many objects has user presets
12. Midi support (supports many controllers) and (if you want) JACK support
13. A built-in Virtual Keyboard which let you to use it even if you don't have a real midi keyboard
14. The graphical user interface can be completely disabled if the user need so
15. It is a free program, licensed under version 2 of the GNU General Public License
16. LASH support
17. DSSI audio output
18. Over 300 high quality instruments included
19. Many other features... It is is impossible to describe all this program's features in just a page

Three synthesizer engines:
1. "ADsynth" or "ADnote" is a complex engine which makes sounds by adding a number of voices. Each one has filters, envelopes, LFOs, morphing, modulation (Ring Modulation, Phase Modulation... the modulators can have any shape), resonance, etc. Each voice includes a very powerful waveform generator with up to 128 sine/non-sine harmonics. You can use Fourier synthesis or if you don't like it you can wave-shaping/filtering of functions. This engine includes anti-aliasing.
2. "SUBsynth" or "SUBnote" is a simple engine which makes sounds through subtraction of harmonics from white noise
3. "PADsynth" or "PADnote" is engine that makes very beautiful pads and other instruments. (these instruments can be exported for use with other programs too)

Effects:
1. Reverb, Echo, Chorus/Flange, Phaser, AlienWah (it's a kind of vocal morpher), Distortion(Wave-shaping), EQ, DynamicFilter
2. The effects can behave as System Effects or as Insertion Effects; this allows to use many effects at the same time. You can put any insertion effect on any part(or more insertion effects to same parts, if you like so) or you can put the insertion effect to Master output (very useful if you use the EQ effect). Also, you can have instrument's effect (which save with the instrument).

A powerful realtime, multi-timbral software synthesizer for Linux and Windows. ZynAddSubFX is microtonal, and the instruments made by it sounds like those from professional keyboards. The program has effects like Reverb, Echo, Chorus, Phaser.

The license of this software is Free, you can free download and free use this music composer software.