Info from Trebor, triaged by Gale, that can be incorporated into this page (add those plugins not already listed).

Mbole Filter (Windows, VST) is a multi-effect plugin inspired by Waldorf DPole Filter. It was developed by MB-PlugIns and now is discontinued. MBole Filter has 4 filter types (LP, HP, BP, Notch, each 12dB/oct or 24dB/oct slope), a ring modulator with 3 waveforms, downsampler, stereo delay, envelope generator and sports 3 waveform LFOs. FL Studio All Plugins Edition for $399. Up to 82% OFF for upgraders. (discontinued) Gross Beat. Windows VST installer: Windows FL Studio installer.

The JS series & MDA series of VST plugins, (which are old) , wouldn't work properly with Audacity 2.1.0 , see https://bugzilla.audacityteam.org/show_bug.cgi?id=1083 for status, it is DEVEL-FIX MADE now.

The “Antress modern” series of plugins, http://antress.blogspot.co.uk/ do work , but since implementation of real-time-adjustment , you have to stop then start playback for any adjustments to the controls to take effect. This is not on Bugzilla, so needs testing but it could have already been fixed by the fix for https://bugzilla.audacityteam.org/show_bug.cgi?id=1084 .

Free VST Plugins are one of the most exciting things about mixing. In the past it was impossible to imagine that so many quality tools could be available at no cost. Today there are more great free plugins than ever before, but the freeware scene is always changing. URS Saturation 64bit plug-in v3.0. Native AU VST RTAS $249.99 Special $99.99. AAX, Audio Units, VST3 and VST Mac OSX support now included!

Free VST effects I have on my machine right now, which I use regularly, so I know are working OK on Audacity 2.1.0:
- Marvel GEQ.dll http://www.voxengo.com/product/marvelgeq/https://forum.audacityteam.org/download/file.php?id=13343]A graphic equalizer
- ANWIDA Soft DX Reverb Light.dll http://www.anwida.com/product.asp?pid=7, http://www.anwida.com/dc.asp?file=http://www.anwida.com/progs/asdxrl20.zip&pid=7
- BC Chorus 4 VST(Stereo).dll http://www.bluecataudio.com/Products/Product_Chorus/
- Gmulti.dll , Ggate.dll , Gsnap.dll http://www.gvst.co.uk/downloads.htm [ compressor, gate, autotune ].
- SPITFISH.dll http://www.digitalfishphones.com/main.php?item=2&subItem=5 [ a basic de-esser ].
- TB_Sibalance_v3.dll http://www.toneboosters.com/tb-sibalance/ [ a sophisticated de-esser ].
- DtBlkFx.dll http://rekkerd.org/dtblkfx/ A spectral-processor [ capable of weirdness like nothing else ].
- 'Acon Multiply' https://acondigital.com/products/multiply/ It does chorus & stereo effects . It has presets, so is simple to operate : ideal for newbies who wants to add effects to vocal / guitar recordings.

This page describes support for VST (Virtual Studio Technology) effect plug-ins in Audacity; VST instrument plug-ins that generate audio are not supported. A list of VST effects known to process correctly in Audacity on Windows is given here.

VST effects are supported by Audacity on OS X and GNU/Linux, but relatively few are available on Mac (due to Apple's competing Audio Units technology) and even fewer on Linux. If a specific VST effect is required on Linux, it may be necessary to employ workarounds such as running Audacity for Windows in a virtual machine.

Related article(s):

Other pages in our Plug-ins category

3List of functional VST Plug-ins

VST support in Audacity

Discontinued Vst Plugins

On Windows and Mac OS X, Audacity is a 32-bit application so won't see 64-bit versions of VST plug-ins, even on 64-bit operating systems.
On GNU/Linux, 32-bit systems will have 32-bit versions of Audacity which won't see 64-bit VST plug-ins. 64-bit systems will have 64-bit versions of Audacity which will see 64-bit VST plug-ins but won't see 32-bit plug-ins.
VST instruments (VSTi) (such as synths) and real-time VST effects (that change the audio data while playing or recording) are not yet supported.
VST 3 plug-ins are not supported.

VST effects are displayed with full GUI interface (where provided by the plug-in), and without need of the VST Enabler. This has been made possible by use of an open source VST header.

For instructions on how to load new VST effects into Audacity please see this page in the Manual.

The overwhelming majority of VST effect plug-ins should display and process correctly. If an incompatible VST plug-in does cause a problem, click the Manage button in the effect's dialog then choose 'Options...'. In the dialog that appears, try reducing Buffer Size, disabling Latency Compensation or disabling graphical display. See VST Effect Options in the Manual for more information on these choices.

To report a plug-in that is problematic in current Audacity, please report it on the Audacity Forum.

Installing VST plug-ins in Audacity

The process and file location is different for each operating system. See the following pages in the Audacity Manual:

List of functional VST Plug-ins

Instructions for adding plug-ins to the list:

Please insert new plug-ins that work in Audacity in the correct place so as to keep the list sorted in alphabetical order.
Please give a download link to the plug-in if possible, enclosing the link inside single square brackets - for example:
[http://www.linktoplugin.com]
If plug-ins are not free, please note this.

The majority of plug-ins will be for Windows only unless OS 9 or OS X is stated.
Classic Mac OS plug-ins will only work under OS 9 and previous. Mac OS X plug-ins with a .component file marked as 'PowerPC' will only work on PPC Macs (they might work on Intel Macs with slow performance under Rosetta but Rosetta cannot be used on OS X 10.7 or later).

List of Plug-ins with download links

Antares AutoTune Evo - Mostly works with current Audacity. Automatic pitch correction works, but to be able to draw pitch curves clock must be set to 'internal' and not 'host'. (commercial)
Antress Modern Plug-ins including a number of compressors, a limiter, an expander, an exciter and a 'spacer' which could be used for creating 'fake stereo'
ANWIDA Soft DX Reverb Light.dll for Windows and Mac (PowerPC only)
Bo Johansen Plug-ins
Bluecat Plug-ins
bs2b.dll stereo to binaural
Buzzroom CL 201 (Vintage Maximiser) - now shareware

Peter 16Nov15: I'm not sure I want us to continue linking to the Brothersoft site (or its plug-ins) the very first ad I got served on that page was for a porn site - sure the adds cycled round with normal sensible ads but they were on a cycle and the porn ad kept returning. And It carries a misleading big green 'Download' button !

Gale 18Nov15: The Classic plugins below are discontinued. The links were pointing to Brothersoft which is a dubious site and now point to VST4free which is much less bad. Also you can get the complete bundle from http://acoustica1.cachefly.net/other/Classic-Effects-Installer.exe. The installer puts the plugins in %PROGRAMFILES%VST where (according to the Manual and VSTEffect.cpp) Audacity should not see them unless a registry key or VST_PATH environment variable points there. But Audacity does see them (on Win 10).

Classic plug-ins Vst

Classic Chorus.dll
Classic Compressor.dll
Classic Delay.dll
Classic EQ.dll
Classic Flanger.dll
Classic Master Limiter.dll
Classic Phaser.dll
Classic Reverb.dll

Peter 16Nov15: Rather than linking direct to the downloads for these digitalfishphones plug-ins, might it not be better to link to the site's download page for them as it tells the user, in some detail, just wh