I'll chip in with some of the free plugins that I've come across that are way too good to be free.
First off there's
Nil's K1v, a 1:1 recreation of the Kawai K1 synth that needs no additional ROM setup and is pretty much plug and play. Doesn't consume too much CPU either.
Everything U-He are always some of the best audio plugins currently available and
Zebralette is basically a stripped down version of their flagship synth Zebra 2. But even with a single sound source it can still sound massive and is pretty much a testing bed for Zebra2. Their whole suite of free plugins are extremely solid and the full versions are decently priced too.
VirtualJV is a wip low level emulation of the Roland JV-880 (the weaker cousin of the JV-1080, which was widely used in the 90s by both film and game composers alike), the thing is pretty rough around the edges currently as right now the only tweakable parameters are the FX section since the Edit tab hasn't been finished yet. But it not only has the factory patches from the 880, it also includes the RD500 digital piano sound set and the gimped but full suite of all the 19 SRJV80 expansions cards, which vary from typical 90s cheesy digital synth sounds to very niche genres like techno, 3 different types of world music and even country. It does require some setup but the ROMs for both the synth and the expansion cards can be found in archive.org afaik.
Osirus/OsTIrus have already been mentioned before ITT but recently the team behind those released a 1:1 emu of the
Nord Lead 2 virtual analog synth. You still need to find the BIOS file separately but it's pretty much the synth in a digital enviroment and can even load patches from the physical unit. The only tibdit about it is that as of now preset names have not been implemented so patch loaded into it will just be named 1,2,3,etc. But I've attached a file that contains all the factory patches Nord released back in the day plus some more for the NL1 in .mid format plus the necessary json files needed so it can actually display the name of the patches.
For some simpler VSTs that don't do much regarding editing capabilities but can still fit in some mixes are
Redtron,
Ticky Clav 2 and
CollaB3. The first is a Mellotron collection of samples dressed in a custom sample player and comes in two flavors: SE, based on the original models people like the Beatles and the Moody Blues used and 400, which is based in the latter models that King Crimson and Genesis used to mention some. Ticky Clav 2 is a lightweight emulation of a Hohner Clavinet, an instrument that is mostly associated with classic 70s funk and Motown, made famous by Stevie Wonder on Superstition and finally CollaB3 is an recreation of a Hammond drawbar Organ, which is pretty much iconic for everything from classic 60's-70's pop to jazz, prog, fusion, etc. Getting the hold of it might be trick at first but searching for a tutorial on how to use a Hammon in general works as well.
A honorable mentions goes to
SWAY, an upcoming emulation of Yamaha's FM/Sample hybrids, the SY77/TG77. As of now the project is still missing some key finctions like evelopes, some samples and the FX section but it's ecpected to go on a provate beta soon in case someone wants to keep an eye for it.
Also, I haven't seen it mentioned yet, but since trackers have been mentioned, some people here might like Renoise. It's basically a modern DAW full with MIDI and VST support out of the box but it's entirely build arround a tracker interface and can even load and export both samples and projects in legacy tracker formats. The demo is basically the full program but with some occasional nag screens and the inability to render your works, but unlike FL, you can actually save and reload your files at a later date.