Plugins are what the name says, external modules that can be plugged into a sequencer program. There are paid and free versions.
Most common formats are AU and VST.

On Apple devices, GarageBand and LogicPro X are using AU plugins, but programs like Reason and Reaper also accept VST plugins.
On Windows platform, all plugins are in VST format.

You will also see nowadays the term MPE (MIDI Polyphonic Expression). MPE compatible means that the synthesizer understands the rich MIDI data sent by special keyboards, such as the ROLI Seaboard keyboard. Have a look at Marco Paris’s cover of ‘Purple Rain’ by Prince – listen all 11 minutes of it!

Almost all these sequencers are available free of charge:

• SoundBridge (registration required, loads slowly)
• LMMS
• Waveform, (requires registration) Good tutorial!
• Reaper, (demo available, full version costs about 60 €)

Almost all of the synthesizers are available free of charge:

• Surge, brilliant although somewhat tricky to use, MPE compliant!
• HELM, good for beginners
• One Button Synthesizer, fun random sound generator
• DEXED, Yamaha DX-7 emulator (FM synthesis)
• Hammond B3 -synthesiser
• Pendulate, mono, unusual concept, MPT compliant
• Exakt lite, FM synthesis for beginners
• OB-Xd, Oberheim emulator
• Tyrell N6, classical sound
• TAL-BassLine, monophonic bass synthesiser
• TAL-NOIzE M4K3R, simple and fun
• Zebralette, excellent synth
• ODIN II, work in progress but is functional and versatile
• AAS Swatches, sound library (requires your e-mail address)

Screen-based modular synthesizers where you have to build your sounds yourself:

• miRack (costs a bit but is available for Apple devices including iPad and iPhone)
• Voltage Modular Nucleus, modular synth, good for study purposes (free, requires registration)
• VCVRACK, Eurorack emulator

Most of the following effects are free

• NOVA, dynamic equaliser
• Cloudseed, reverb, only Windows
• ValhallaSupermassive, reverb
• FreeClip, limiter / wave transformer
• IVGI2, saturator (enricher of timbre)
• MJUC JR., compressor
• Leveling Tool, vocal compressor
• SGA1566, mic preamp emulator, saturator, etc.(warning: processor-intensive)
• Cypress TT-15, guitar amp emulator
• Convology XT, convolution reverb
• PAULXSTRETCH, time-stretcher (granuliser), AU version no longer works in Catalina!
• Fracture, delay etc.
• TAL-Vocoder, vocoder
• Signalizer, oscilloscope, AU version no longer works in Catalina!
• MeldaProduction, various plugins