Free VSTs

These VSTs are worth having in every arsenal, despite being FREE. I've left out a few that I use because Ableton has better internal options

I'm listing the ones I use, but here are a few good lists if you're looking for more
KVR list
LANDR list
theproaudiofiles.com list
thehomerecordings.com list

I also recommend picking up a copy of Computer Music (online or mag), you will get a GIANT backlog of amazing VSTs for free!

Packages or Companies with several products
GVST
Several great effect plugins & Synths

Melda
Free Effects, and amazing plugins

Native Instruments - Komplete Start
Synths, Effects, Instruments

Plugin Alliance
Effects

Reaper - ReaPlugs
Plugins from Reaper DAW as VSTs

TAL
Effects & Instruments (Older plugins may require a 32bit vst bridge like JBridge)

Voxengo
Awesome Utilities and Effects


Analyzers
Melda - MOscilliscope
s(M)exoscope
Voxengo - SPAN

Auto-tuners
GVST - GSnap
Melda - MAutopitch

Bit Crushers
Melda - MBitFun

Clippers
Venn Audio - Free Clip
GVST - GClip (Windows Only)

Compressors
Klanghelm - DCA1
Klanghelm - MJuc Jr (Mu Compressor)
Melda - MCompressor (Adjustable Compressor)
Xfer - OTT (modeled after Abletons OTT Multiband dynamics, but still useful)

Filters
Melda - MComb (Comb filter)
Plugin Alliance - bx_cleansweep (analog LP/HP)

Glitch
Illformed - Glitch 1 (may require JBridge)
GlitchMachines - Fracture
GlitchMachines - Hysteresis (glitch delay)

Instruments
Dexed - Yamaha DX7 clone
Helm - Free, but excellent synth
Spitfire Audio - Labs (Excellent Sampled Instruments)
VCV RACK - The mother of Free modular synths
You will need to install 0.6.2c version before the latest to get the Vst bridge - available here
VeeSeeVST Rack - A pre-built vst of vcv rack with several free modules included
Wavosaur - VST Speek (8-bit speech engine)

Mix
Plugin Alliance - bx_subfilter - Mix tool for your sub chain

Reverb
Melda - MConvolutionEZ (Convolution Reverb)

Saturation
Klanghelm - IVGI
Softtube - Saturation Knob
Voxengo - Tube Amp

Stereo Imaging
Izotope - Ozone Imager 2
Polyverse - Wider (My fav plugin for generating width)
Voxengo - MSED Mid/Side Adjuster
Xfer - Dimension Expander (chorus spatial expander to emulate NI Massive dimension expander)

Timestretching
Xenakios - PaulXStretch (Extreme time stretching)

Vinyl Effects
Izotope - Vinyl

Vocal Effects
Izotope - Vocal Doubler