1 Engines:

ZynAddSubFX (Polyphonic Aditive, Substractive, and 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
Pianoteq (A proprietary Physical Modelling Synth Engine for acoustic and electric pianos, developed by Modartt)
Aeolus (Pipe Organ Emulator) https://kokkinizita.linuxaudio.org/linuxaudio/aeolus/
Jalv (LV2-plugins as standalone layers) http://blog.zynthian.org/index.php/2018/09/28/new-engine-jalv-lv2-plugins-as-standalone-layers/
MOD-HOST & MOD-UI (Plugin Host + Web GUI, developed by ModDevices for the ModDuo device) [1] [2]
Pure Data (Visual Programming Language) https://puredata.info
CSound (Sound Programming Language) https://csound.com

2 LV2 Plugins: Synths & Effects

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 that includes OBXd and others)
DISTRHO DPF-Plugins (A plugin collection which includes the Nekobi Bass Monosynth and more)
Calf plugins (One of the best free software plugin collection)
Synthv1 (Old School Polyphonic Synthesizer)
Samplv1 (Old School Polyphonic Sampler)
Drumkv1 (Old School Drumkit Sampler)
Padthv1 (Old School Polyphonic Additive Synthesizer)
Helm (Advanced Polyphonic Synthesizer + Arpeggiator)
SooperLooper Live looping sampler capable of immediate loop recording, overdubbing, ...

Zynthian include all kind of effects:

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

we have not a complete list of all synths & effect plugins, but you can take a look to this for having an idea:

Currently, Zynthian supports only LV2 plugins compiled for RBPi (ARM):

Here is an unmaintained repository of LV2-plugins for Zynthian: https://github.com/zynthian/zynthian-plugins/tree/master/lv2

Support for VST, DSSI and LADSPA has been removed for simplicity, but it could be re-enabled in the future if we have good reasons for doing so.