REAPER
Reaper is the most widespread DAW in the Ambisonics community.
Not only it is cheap and highly customizable, but it is the only DAW currently available that supports up to 64 channels in a track, making high order ambisonics possible up to the 7th order. Moreover, the signal routing is very flexible and allows very complex setups.
IEM Plugin Suite
A free and open source plugin suite for Ambisonics developed by the Institute of Electronic Music and Acoustics at KUG in Graz. It's capable to process Ambisonics up to the 7th order and features a wide variety of Encoders, a powerful AllRAD decoder and some signal processing plugins, such as a multichannel compressor, a reverb and an EQ.
ambiX plugin suite by Matthias Kronlachner
A free plugin suite featuring cross-platform (and very well sounding) ambisonics encoders and processors in first, third, fifth and seventh order.
One of the advantages of these plugins is the possibility to adjust the channel weighting. On the other hand, when using multiple sources, their positioning is not extremely flexible.
The Ambisonics Toolkit (ATK)
The ATK is a plugin suite developed by Joseph Anderson and originally implemented in SuperCollider. It was recompiled as JSFX's for Reaper by Trond Lossius.
ATK features a number of extremely interesting and powerful spatial transformation tools, but it is important to know that these tools are limited to First Order Ambisonics and follow the FuMa channel numbering/normalization convention, therefore - if using the AmbiX convention it will be necessary to convert the signal. The ambiX plugin suite has FuMa/Ambix converters, so fear not. Remember also that FOA can be upmixed to HOA using the Sparta Upmixer plugin.
ATK has just released a major update that makes it capable to handle High Order Ambisonics, but it is for Super Collider only at the moment.
SPARTA
Sparta is a project of the AALTO university that resulted in different free plugin suites covering different aspects of the Ambisonics workflow.
The plugins are very complex but also extremely powerful, and while the learning curve is sometimes a bit steeper than with other tools, the results are often very rewarding. So, if you feel like reading a bit, this is a great set of tools to explore.
Their upmixer is to be found in the Compass bundle.
MELDA MFreeFXBundle
This free Plugin Bundle is more oriented towards music production and represents therefore a great addition to all the other great tools for multichannel audio available out there.
Most of the FX's here support Ambisonics, but it needs to be activated. To do so, click on the Toolbar button on the right of the VST window and then the button indicating the channel setup (by default this is set on L+R). This will open a new window where you can tick the box saying "activate ambisonics". You will then require to restart your DAW but once done you will have plugins supporting up to H7OA 3D ambisonics.
x-mcfx convolver by Angelo Farina
This convolver is a fork of the mcfx-convolver by M. Kronlachner, it features improved performances and is recompiled both as VST2 and VST3. In addition to that, Angelo Farina provides a number of filter matrixes from different HOA microphones, such as Zylia ZM-1, useful to encode HOA A-Format recordings into B-Format.
http://www.angelofarina.it/X-MCFX.htm
The filter matrixes for Zylia ZM-1 are to be found here: http://www.angelofarina.it/Public/Zylia/
SPAT5
A very wide and omnicomprehensive MaxMSP library for spatialization and ambisonics. It is developed by IRCAM and now available for free. Spat also offers very good and detailed documentation for every object with good examples patches showing signal routing and workflow. Moreover it sinergizes well with the mc. wrapper from Max8.
Keep in mind that everything done in SPAT presents a 90° azimuth offset compared to IEM. So, if working with both, remember to rotate the soundfield.
Blue Ripple
A plugin bundle oriented towards Gaming and VR other than Pro Audio, it supports up to 3rd Order Ambisonics and is very user friendly.
The core bundle of Blue Ripple [O3A Core] is free and provides some quite interesting beam forming tools using the Beamer or the Virtual Microphone, as well as a stereo decoder.
https://www.blueripplesound.com/
Link to the free bundle:
https://www.blueripplesound.com/products/o3a-core
GRM Tools
GRM Tools is a package offering great sound processing tools for stereo and some tools for multichannel applications. GRM SpaceGrain especially is a very powerful granular synthesizer capable to output up to 32 channels representing coordinates of virtual speakers. The softwares are already equipped with a few presets of speaker layouts, but new layouts can be created.
GRM Spatial Tools do not support ambisonics, but the different channel streams can always be re-encoded in a IEM multiencoder for instance.
Soundfield by Røde
This plugin is intended to be used in combination with the Røde NT-SF1 microphone, and can either yield B-Format Ambisonics or a number of different audio formats using virtual microphones.
Røde also claims to have left behind the "matrices and correction filters of previous generation" to adopt a new frequency-domain processing technology (suggesting they are using something closer to a DirAC or a parametric reprodution method, thus undocumented).
Sennheiser dearVR and AMBEO
The plugins offered by Sennheiser can be thought of commercial production oriented plugins, recreating 3D virtual environments that are meant for immersive music production or VR applications. Of those, only AMBEO Pattern and dearVR Micro/Ambi Micro are free and are meant to be used in combination with the Sennheiser Ambeo VR Mic.
VVStandard and VVHoa by VVAudio
These encoder/decoder bundles are meant to be used with CoreSound Tetra/Octo mic or with Brahma microphones. Buying a CoreSound mic will also produce a coupon code to download VVStandard or VVHoa. These encoder can otherwise be independently purchase and will accept different calibration files.