I've compiled this list quite some time ago and often share it in comments whenever someone seeks recommendations for free plugins. Now, I feel it deserves its own dedicated post. This selection includes both widely recognized and lesser-known plugins, all of which I have personally tried and can attest to their quality. I hope you find it useful!

Synths:

1 - Helm : A very visually-oriented and user-friendly subtractive synth. Its biggest advantages are its 3 assignable LFOs and envelopes, its step sequencer, and its keytracked comb filter. You can get sounds out of this thing that'd be impossible with really any other plugin, but it's also a very capable general-purpose subtractive synthesizer.

2 - Synth1 : An analog emulation synth based on the Nord Lead 2 (popular hardware synth from the 90s). Its greatest advantage is its presets - there's a free bank with 25,000 of them. And you can also get the free Synth1 librarian add-on to manage your 25,000 presets.

3 - Spitfire LABS : A host for several free instrument libraries, with new ones added on a regular basis. Perfect for orchestral sounds, or other recorded instruments that are hard to find for free.

4 - DSK's assorted instrument plugins, such as Dynamic Guitars, Brass, Overture, and Strings aren't on the same level as $500 Kontakt libraries but they sound fairly good with the right processing. Use these alongside LABS and you'll be all set for making orchestral music.

5 - Ample Bass P Lite : Free bass guitar plugin. Sounds good with saturation.

6 - Keyzone Classic : Piano plugin. If you use the Yamaha Grand Piano preset, this is probably the best-sounding free piano plugin you can get.

7 - BlueARP : Not an instrument, but a midi arpeggiator.

Effects:

1 - OTT : A multiband compressor based on a well-known Ableton multiband compressor preset of the same name. It's a big part of the "EDM sound", put it on anything from basses to leads to make them sound tight, bright, and compressed.

2 - Ozone Imager : A stereo imager that is actually good. Has a stereo visualizer, and only two controls - Width, which increases or reduces a sound's existing stereo width, and Stereoize, which adds mono-compatible stereo width to sounds using a stereo-phase-inverted delay.

3 - dblue.Glitch : A step-sequenced glitch plugin. It's up there alongside Gross Beat and Halftime as one of the best ways to add weird time-based effects.

4 - dblue.TapeStop : Simulates the downward pitch slide and volume reduction caused by stopping a tape player. Useful in nearly every genre. Only two knobs: one is automated to trigger the effect, the other controls the length.

5 - CamelCrusher : A compression/distortion plugin perfect for fattening up basses. More versatile and customizable than sausage fattener too.

6 - Krush by Tritik : Basically the best bitcrusher you can get, full stop. The usual quantization and sample rate reduction, as well as distortion, a filter, and an assignable LFO.

7 - PanCake : Super customizable auto-panning plugin. Simply draw your own LFO shape, set the length and mix level, and you have all the pan automations you could ever need.

8 - NC-17 : Soft clipper/ Chebyshev filter plugin. Can make things loud and warm, or harsh, dirty, and distorted - without destroying the base sound. Good for everything from bass design to mastering.

9 - Saturation Knob : Super simple 1-knob saturator, great for adding a little bit (or a lot) of analog warmth/grit to sounds.

10 - SPAN : Really good spectrum analyzer. Great for checking mixes, and can be used alongside Parametric EQ 2 for more precise EQing.

11 - Izotope Vinyl : The essential lofi plugin. Simulates playing stuff on an old record, and has a bunch of settings to adjust the character of the simulated wear and tear.

12 - Voice of Snow : Lets you independently control the volume of the noise and tonal elements of a sound. Great for processing samples and denoising recordings.

13 - the Melda Production FreeFX bundle : A bundle of free plugins with a ton of really good effects. And it's not just the basic stuff, this pack includes a pitch correction plugin, a frequency shifter, a comb filter, a ring modulator, and a bunch of other cool stuff.

14 - Transpire by Sonic Anomaly : A very capable transient processor, simple to learn and use with a sleek GUI. Great for modifying drums, plucks, and everything else with a transient.

15 - TAL-Filter-2 : Draw your own LFO to control filter cutoff or volume. Poor man's Kickstart/LFOTool.

16 - Ignite Emissary : Harsh, dirty guitar amp simulation. For when ordinary distortion just isn't rough enough.

17 - Pitchproof : Pitch shifter/harmonizer. Make sure to put it in stereo mode.

18 - Kilohearts Free Toolbox : Contains 6 simple yet useful plugins - a 3-band EQ, a delay, a chorus, a gain control, a stereo control/correlometer, and a limiter. None of these plugins have any special features that the stock FL plugins don't have, but their well-designed minimalist UIs make them very fast and efficient to use.

19 - D16 Frontier : Analog limiter + soft clipper that automatically adjusts gain. A quick and easy way to bring tracks up and add warm analog grit at the same time. Sounds great on drums.

20 - Glitchmacines Fracture : Glitch FX plugin. Includes a buffer/sample&hold, a delay, and a filter, each with its own LFO. Great for glitchy textures and fills.

Samples/Presets:

1 - Cymatics Free Download Vault : A bunch of high-quality free samples and presets, all in one place.

2 - SampleFocus.com : These aren't generally as good as the Cymatics samples, but it's not a bad place to find those specific odds and ends that you can't find in the Cymatics packs.

3 - Polysonic 10 free dubstep snares pack : This is just some random small sample pack, but I use these snares all the time. The dubstep snares in the Cymatics packs just aren't as good as these.

4 - 99sounds : They have some good free ambience sample packs.

5 - TheMetalKickDrum Vortex kit : Excellent kicks, snares, and toms for metal.

Don't forget to check out Tunebat Marketplace for all your music production needs. Happy remixing!