Simple Microtonal Synth is a free microtonal polyphonic native 64-bit VST Windows synth from Michael W. Dean of BipTunia.
Walk-through video is here.
Simple Microtonal Synth is robust. It is also the easiest way to get into making microtonal music, period.
Simple Microtonal Synth has 32 pre-sets, and 5378 microtonal tuning options.
This synth is great for heavy bass, forceful leads, crystalline chimes, and more.
TIP: Don't be afraid to experiment with microtonal music if you don't yet understand the math. While eventually learning the math is good, and we included resources for that. And with this VST, no math is even needed to start making cool microtonal music. In fact, an ability to create compelling music with a tiny bit of understanding of microtones will go much further than the opposite. Simple Microtonal Synth is a very musical homage to vintage analog synths.
Simple Microtonal Synth is fun, beautiful, and as simple as can be.
(There is also a 32-bit version here.).