This post is for the Mac users.
When it comes to Mac OS X, the number of freeware plugins is very low, compared to the Windows world.
The popular “Save as VST” softwares (Synthedit and Synthmaker) are Windows only.
The “upgrade” politic of Apple is a sort of nightmare for developers (and sometimes users!), so many old free plugins don’t work anymore…

I have searched for free MAC plugins over the web and then tested !
– only free synth (not effects), in VST and or AudioUnit (AU) format
– the OS used is Mac OS X 10.8 (Mountain Lion)
– i tested the plugins in Ableton Live 9 / Renoise / some free VST & AudioUnit host softwares (Pedalboard, the JUCE open source plugin host, VSTLord, AULab, Ugly VSTi Interface…)

What are VST Plugins? The world of virtual software instruments is an incredibly complex, far reaching landscape, ranging from near-identical iconic vintage synthesizers to futuristic, endlessly versatile programs, and everything in between. Free VST downloads: TN-5 (Yamaha CS-5 emulation by TN Audio) - Transcendental 2000 (Powertran Transcendent replica by TN Audio) - Syncla-X (Synclavier FM emulation by Digital Systemic Emulations) - Odyssey (ARP Odyssey emulation by Synth School) - Supersaw OSC (Roland Supersaw osc synth by CFA-Sound) - easy-pro-six (SC Prophet based synth. VST 4 FREE - Free Audio Plug-ins and Archives. Free VST downloads: TN-5 (Yamaha CS-5 emulation by TN Audio) - Transcendental 2000 (Powertran Transcendent replica by TN Audio) - Syncla-X (Synclavier FM emulation by Digital Systemic Emulations) - Odyssey (ARP Odyssey emulation by Synth School) - Supersaw OSC (Roland Supersaw osc synth.

It’s a selection with free virtual analog synths, organ emulations, epiano emulations, drum synth, bass synth, FM synth, emulation of Oberheim / NordLead / DX 7 synth, chip synth, guitar emulation…

and here a list of 25 free synth plugins for Mac OS X :

1. Synth1 AU/ VST

The must-have free synth plugin is available for Mac since one or two years, it is in beta state, it still have some minor bugs with the UI, but it’s usable and you can benefit from the incredible amount of presets created for Synth1, including these 😀
=>
https://blog.wavosaur.com/synth1-presets-chris-bank-volume-2/ &
https://blog.wavosaur.com/chris-bank-synth1-reup/
I had some problems for saving preset in Ableton Live.
Synth1 can do an incredible variety of sounds.
The most versatile Synth in this list

2. OBXD AU / VST

OBXD AU

The free Oberheim OB-X OB-Xa OB-8 emulation !
work without any problem in my AudioUnit / VST hosts on Mac.

3. Tyrell N6 AU / VST

http://www.u-he.com/cms/tyrelln6
DL here : http://www.amazona.de/amazona-de-freeware-synthesizer-tyrell-n6-v3/

U-He loves the Mac, don’t hesitate to have a look at the other freeware on the Website, all are working without problems in VST or AudioUnit format. I have also selected Triple Cheeze and Zebralette from the same developer. These plugins are good and full featured.

TyrellN6 plugin comes with a big amount of presets, and a deep variety of sounds. It’s powerful and easy to program. It’s rock solid stable and have a nice UI too!

4. FreeAlpha AU / VST

FreeAlpha AU

This freeware from LinPlug sounds very good, it has punch and is easy to tweak. It can do punchy bass, but also nice pads and clean leads. Despite its simple look, it’s powerful and funky!

5. Crystal AU / VST

The old Crystal Synth is still top, and one of the best synth for complex / evolving PADS, with its multipoint envelopes, and the complete modulation matrix.
One of the best virtual synth, for more than 10 years!

6. TAL NoiseMaker AU / VST

TAL Noisemaker

Like U-He, TAL has many freebees for Mac OS X.
I have selected Noize M4k3r and Elek7tro but you also have other interesting plugins.
The “Noise Maker” plugin has two osc + sub, a good filter and the famous Chorus “a la Roland”, so you have all you need for your fat bass/lead. Certainly more agressive than Crystal, it completes well the sonic arsenal.

7. Dexed FM VST

The famous free Yamaha DX7 emulation ! It’s VST only (no AU for the moment).
Use the cart / load / save button to browse or import presets.
It can read / write sysex files for and from the DX7.
Now all your FM needs are fullfilled: you can do pop ballad or Detroit techno, vive la FM !

8. MiniSpillage AU (Drum Synth)

MiniSpillage AU

Now that we have the FM synth, the Pad synth, the Bass synth and Lead capable synths, we need a DRUM synth. The Minispillage plugin has three dedicated algo for bass drum, wood drum and hihat.
I tend to prefer “Drumatic” but Drumatic is Windows only.

Vst Plugins Synth

9. ComboF organ AU / VST

After all these synths, we now need an emulation of our prefered organ : the Farfisa combo organ!
ComboF gives you the sound of the Italian electric organ we all love. Available for Mac and windows, in AudioUnit and VST!

10. MrTramp 2 e-piano AU / VST

MrTramp2

After the organ emulation, we need the electric piano! MrTramp 2 is the best (free or not) emulation of the Wurlitzer electric piano. It’s raining again ! Now you can sound like Supertramp ! Breakfast in America !! There are not so much controls but you don’t need more, and the sound is excellent.

11. Cheeze Machine AU

A dedicated string synth plugin for Mac ! The Cheeze machine from Big Tick is a kind of Arp Solina emulation. It has this phaser / chorus creamy sound.
it’s AU only and should work with your favorite Mac host software.

12. TripleCheese AU / VST

TripleCheeze plugin

Cheese again !
The U-He contribution to the first KVR Developer Challenge. It’s an original synth with unusual sound generation. Nice for plucked sounds. Works well on my Mac OS X Mountain Lion. Thanks to the comb filter, you can generate very original sounds.

13. Zr3 organ VST

We have ComboF for the Farfisa emulation, and for the Hammond B3 organ lovers, there’s Zr3.
It’s VST only and works nice in Ableton Live 9.
The only good free Hammond B3 emulation for Mac OS X.

14. TAL Elek7ro AU / VST

TAL Elektro

A perfect free synth for bass and perc sounds.
Don’t forget to check the other TAL plugins !

15. VST Speek AU / VST

Coming from here ! the funkiest text to speech in plugin format !
The best C64 Sam oldskool text to speech emulator, for all your techno voices.
Available in 32 & 64 bit, VST & AU

16. Zebralette AU / VST

Zebralette plugin

Coming with the demo of Zebra, the famous synth by u-He, Zebralette is a nice synth, with unusual sound, and powerful possibilities and plenty of special waveforms to play with.

17. Model – E VST

The antic VST from Steinberg, now free, and now available in VST format for Mac OSX.
It has been surpassed in all domain by many other synth (freeware included). But it can be useful and you can get some nice sounds out of it if you tweak it with love.

18. VB-1 virtual bass VST

VB1 bass

Another prehistoric virtual synth from Steinberg, this virtual bass plugin is free and available with Model E in the same package (see link above). Very simple sound, can be useful.
Freeware for Mac are rare so we take it!

19. MDA Piano ePiano DX10 & JX10 VST

NO GUI

These free VST work for Mac OS X, they don’t have UI, you’ll have to use the default interface supplied by your host.
– Piano & ePiano are sample based synth, the ePiano is very good, it’s a kind of Rhodes emulation.
– DX10 is a FM Synth, it has only two operators, but you can still get nice bell sounds out of it.
– JX10 is a very nice virtual analog synth, despite some bugs (the Windows version has the same), it can do some very nice leads.

20. 4Front R-Piano AU / VST

Vst Plugin Synthesizer

Another free electric piano emulation for your Mac!
It’s simple and has a nice clean sound. Perfect for deep house chords.

21. Chip32 AU

Chip 32 AU

This is the URL where you can also find the Cheeze Machine VST, and also the very good Clavinet emulation by Big Tick (TickyClav). Chip32 is a very simple plugin, perfect for your chip tune needs.

22. Spicy Guitar AU / VST

A free Audio Unit / VST emulation of an acoustic guitar ! It’s use a physical modelled synthesis, and is a great emulation. Here are coming the ukulele / banjo / Flamenco guitar !

23. Scythe VST

Scythe VST

A very simple VA Synth (VST only), with two oscillators, a filter, modulation envelope & some embedded effects : bitcrusher / phaser / chorus / delay.
It’s simple but efficient.
You can also generate random presets!

24. Automat AU

A virtual substractive synth with 3 osc, and filter for each osc, + many effects and modulations.
It’s audiounit only, and like Scythe, it has a random preset generator.

25. VOPM VST

Free 64 Bit Vst Plugins

VOPM VST synth

A VST emulation of the Yamaha YM2151 Chip that was used in many arcade games, and is similar to the chip in the Sega Megadrive. It’s a 4 operator FM synth, with 8 algorithm.
here’s a lot of preset for VOPM : http://truechiptilldeath.com/blog/2010/05/04/opm-patches-out-of-most-genesis-games/

26. Bonus tracks ?

1) there’s also kickmaker, a free kick drum synthesizer in VST and AU format : http://teragonaudio.com/KickMaker.html

and C700, that can load sample in aiff and wav format :
http://picopicose.com/software.html

2) I also tried other free VST plugins for MAC , but with no luck (no one working in any host on my Mac OS X Mountain Lion), i think these are too old plugins, or maybe ppc plugins (even if they were said to work on Mac OS X).
For example :
the Delay Lama ! http://www.audionerdz.nl/download.htm
AU303 & AirySynth : http://airy.andre.online.fr/AU/index.html#au303

3) I’m working on a port of Rave Generator VST for MAC, here you can find a beta (x64 only, VST only) :
https://www.facebook.com/groups/149376535107031/835302689847742/

3) I’m also working on a port of CreakBox (the TB303 plugin) for MAC OS X (i already have converted it to VST2.4 then to 64 bit), see link ( @ 6. and in the comments) :
https://blog.wavosaur.com/best-free-tb-303-emulation-sotware-vst-plugins-standalone/

Vst Synth Plugin Ableton

4) i’ll do another posts with the free effect plugins for MAC ! It seems there are more effects available for free than synths.