Here is the alphabetical list of free useful plugins and free plugin compilations:
BBC Symphony Orchestra - free, easy-to-use plugin, with 33 instruments brought to life
–> Create a new account, fill out the questionnaire, and get the free download after 14 days
CollaB3 - a free vintage tonewheel organ emulation
Dead Duck Software - the Dead Duck free plugins provide all you need to get started making music on your PC
Dexed - a multi platform, multi format plugin synth that is closely modeled on the Yamaha DX7
Full Bucket - 20 free plugins
Helm - a free polyphonic synthesizer
IVGI - a free saturation & distortion plugin
LoudMax - a Look-Ahead Brickwall Loudness Maximizer with a clean transparent sound
MFreeFXBundle - the Melda production free plugin bundle; these quality plugins can compete with paid ones
Noiz-Lab LE Mastering Bundle - a bundle of 5 free mastering plugins in VST, VST3, AU and AAX format for Mac and PC
OrilRiver - a free algorithmic stereo reverb that can rival the quality of commercial reverbs
PanCake - a free plugin for most flexible panning modulations
QRange - a free IIR linear phase EQ
ReaPlugs - a free package of FX that includes many of the plug-ins that come with REAPER
Surge - an open source digital synthesizer
TAL-NoiseMaker - a free synth that also includes a small effect section with a reverb, chorus and a simple bit-crusher effect
TDR Kotelnikov - a free wideband dynamics processor combining high fidelity dynamic range control with deep musical flexibility
TDR Nova - a free parallel dynamic equalizer
TDR VOS SlickEQ - a free mixing/mastering equalizer
Unlimited - a free loudness maximizer and limiter for stereo and 5.1 surround material
Voxengo SPAN - a free spectrum analyzer plugin
Check out other 15 free plugins
Youlean Loudness Meter - a free loudness meter that can compete even with the most expensive ones