1 Engines:

ZynAddSubFX (Polyphonic Aditive+Substractive+Pad synthesis) https://github.com/fundamental/zynaddsubfx
FluidSynth (SF2 SoundFont Engine) http://www.fluidsynth.org
LinuxSampler (SFZ/GIG Advanced SoundFont Engine) https://www.linuxsampler.org
setBfree (Hammond B3 electromechanical organ Emulation) https://github.com/pantherb/setBfree
MOD-HOST & MOD-UI (Plugin Host + Web GUI, developed by ModDevices for its extraordinary ModDuo device) https://github.com/moddevices/mod-host https://github.com/moddevices/mod-ui MOD-UI]
Pianoteq (It's a propietary Physical Modelling Synth Engine for acoustic and electric pianos, developed by Modartt)

2 Plugins that work with MOD-UI

Dexed (Yamaha DX7 emulation)
MDA plugins (LV2 port of MDA plugin collection, including ePiano, JX10, DX10 and others)
TAL free plugins (Some plugins from Togu Audio Line, including Noize M4k3r and more)
DISTRHO Plugins-Ports (A collection of plugins ported to Linux)
DISTRHO DPF-Plugins (A plugin collection that includes Nekobi Bass Monosynth and more)
Calf plugins (One of the best free software plugin collection)

Category	Includes
Delay	Delay, Echo
Distortion	Overdrive, Distortion, Fuzz, Wave-shaper
Dynamics	Compressor, Limiter
Filter	Filter, Equalizer, (Auto-)Wah, Envelope Filter
Generator	Synthesizer, Piano, Organ, Drums, Noise, SoundFonts
Modulator	Chorus, Phaser, Flanger, Vibrato, Tremolo, Ring modulator
Reverb	Reverb, Shimmer, Plate, Room, Spring
Simulator	Amplifier, Cabinet, Leslie, Tube
Spatial	Panner, Auto-panner, Stereo field manipulator
Spectral	Pitch-shifter, Autotune, Harmonizer, Vocoder
Utility	MIDI processing/generating, Volume, Looper, Switcher

You can see the full list in the plugins repository:

https://github.com/zynthian/zynthian-plugins