On the subject of FM synthesis and wandering a little off topic the iOS synth FM4 is a really easy to program FM synth. Four operators rather than the DX’s six but it’s laid out really well visually and can do the FM bell, chime and pad sounds very well.

It’s also very inexpensive, so long as you have an ipad of course.

In hardware the Volca FM and Yamaha Reface DX also seem to have good interfaces. Mind you it’s not hard to improve on the DX 7’s incredibly user-hostile minimalist single display and data slider plus a bunch of buttons approach.

bitflipper
OBXD is a pretty good Oberheim emulation, easy to use and 64-bit. Not a lot of presets available, but it's so easy to program you really don't need them.

This is cool, I didn't know about this. Lacking modulation matrix leaves it bit short for my use, but I've installed it. Meant to be CPU friendly and not having an FX section is a plus for me. 

Not sure if you're looking for synth-ire stuff or more natural, but I can say I'm a HUGE fan of these three,
 
TAL-NoiseMaker which I use in almost every single project, extremely simple controls yet loads of possibilities
Retronix, haven't used it in a while but loads of variety here too especially in the presets,
Superwave P8 which I used to use constantly because it has I think 6 or 8 oscillators all of which can be set to 5 or 6 different waveforms so you can create these insanely dense, wide crispy pads and synth FX
 
I've had trouble finding SO many good ones that provide "natural" sounds, but if you're looking for VST's in general I'd also recommend looking at Saffron SE with is for sound fonts (hundreds of decent, useable and even good ones out there) and ShortCircuit2, arguably the best free sampler out there. Wins out against TXW16x in my opinion in ease of use vs. possibilities

Free synths? I also like OB-Xd and Dexed mentioned above. But then there are PG-8X and Latte(unfortunately 32-bit only). Those ones have also real good sounds.
 
On the other hand if you do not like to invest too much time in creating your own patches (like me), then I recommend to buy Arturia's Analog Lab. It includes hundreds of patches of most of the great analog hardware synths of the past, I really like it!
 
Marc

This is probably my favorite freebie.  It's a Chinese flute.  It sounds great.  I use it often.  I love it.
http://www.vst4free.com/free_vst.php?id=309
 
John B.

I would say for the sheer number of free patches available, it is hard to beat Dexed (based on Yamaha DX-7) and Synth1 (based on Nord Lead).
 
Synth1 has about 15,000 free patches available, and Dexed can load any DX-7 patches.
 
The Amplesound P-Bass and Acoustic Guitar that Kamikaze mentioned are excellent, as well as the free Addictive Keys
Grand Piano (only 4 octaves, but works very well on a 49 key controller)!
 
The OB-Xd is a must have Oberheim emulation!  No on-board FX, but that's what FX chains in your DAW are for!
 
The free synths from U-he (designer of Zebra, Diva, and Hive), such as Zebralette and Tyrell N6 are also must haves!
 
There are also free versions of Native Instruments Kontakt and Reaktor, as well as free versions of IK Multimedia SampleTank 3 and Syntronik.  All of these each come with a limited factory library with no expiration, so check them out!
 
For free drums, I think that MT Power Drumkit 2 is a good go-to option. https://www.powerdrumkit.com/
 

Yikes!  I posted a link to warez site!  I thought it was legal, honest.  I've deleted it.

jude77
Yikes!  I posted a link to warez site!  I thought it was legal, honest.  I've deleted it.

 Funny   That was a good one!!!

One I've used several times is Crystal which is free and 64 bit.  It can be found here: http://www.greenoak.com/crystal/dnld2.html
 
It has a lot of tempo based f/x that really swing and can add movement to a song.

Tyrell N6, new version 3, is a substractive synthesis virtual instrument which is very complete for a freeware.

It features 2 oscillators (sine/tri/saw/square for the first one, saw/square for the second) plus a sub (square), plus a noise generator (white or pink). To modulate the oscillators, there are a vibrato, a “pulse width” (which only affects the squares), a soft sync (which only works if the oscillators are detuned), a ring modulator with feedback. Oscillator 2 can be detuned. There’s a portamento, and that of oscillator 2 can be set offbeat for strange results.

There’s a ringing filter that can be BP or LP/HP (you can get gradually from LP to HP using a MixSpread pot, providing unusual effect sounds in the in-between positions). Slope can be set to 12, 24 or 36 dB-per-octave. Great possibilities of filter modulation.

Then there are 2 LFOs which parameters themselves can be modulated, 2 ADSR envelopes (which can also be modulated) and a matrix. The latter allows to apply a frequency modulation from oscillator 1 to either oscillator 2 or the filter, and to route two controllers to a same parameter (with one modulating the other).
Finally, there is a single effect – a chorus, which is well enough. To finish with this overview, let’s mention that Tyrell can work in mono, legato, duo, poly and unison 2-to-8 (detonable) voices modes.
As far as I know, there’s no manual for Tyrell N6. I regret it as I would have appreciated some precisions regarding some, say, “original” functionalities such as the Drift (an analog model emulation perhaps?) and Fallrise (on envelopes, that has the sound back up after decay).

This freeware is very stable.

All in all, we get a synth that’s quite powerful for a freeware. The ring modulator, feedback and FM on the second oscillator allow to get rather original sounds. But what makes this Tyrell interesting also are its modulation and control allocation capacities (by the way, all parameters can be MIDI-controlled using the MIDI learn feature). Hence, you can get beyond the “factory” 500 presets.

Tyrell N6 is one of the most complete freeware synths you can get on a Mac (but of course it also works on PC). Of course, if you already own Absynth, Massive, Predator, Gladiator or others, Tyrell may not bring you much. Still, even in such a case, there’s always use for a small, quick-to-program synth such as this of Elek7ro. Anyway, a penniless homestudist will always find such synths very useful as their quality makes them a really great deal.
By the way, Tyrell’s maker U-HE also offers numerous great products such as the Zebra synth or the great Uhbik plug-in series. As for Tyrell, it’s available here:
https://www.amazona.de/amazona-de-freeware-synthesizer-tyrell-n6-v3/