ModulAir is a modular polyphonic software synthesizer for both Mac and PC from Full Bucket Music. The list of features include:
Fully modular architecture
Up to 18 modules per patch
43 module types available
Polyphonic Voice and monophonic Master modules
Up to 64 voices polyphony
External signal processing possible
TUN/SCL micro-tuning file import
MIDI Learn – all parameters can be controlled by MIDI CC
Double precision audio processing
Plug-in supports Windows and macOS (32 bit and 64 bit)
VST and AU plugin formats
It sounds great, is a lot of fun to play around with, and is free/donationware. Download it here.