You’ll find lots of free patches for numerous apps (mostly iOS) at Patchstorage .
Free Windows 98 themed skin for Serum.
Roland TR-808 and TB-303 emulation in your browser.
DCAM FreeComp is a free plugin for premium circuit-modelled compression in VST, AU or RTAS hosts. Needs a user registration at FXpansion.
Lot’s of free soundfonts/samples to download from Flame Studios. These are allowed to be used in your productions.
Tonetta Blue is a free semi-modular 8-voices polyphonic synthesizer for VST3 (64-bit)
FrozenPlain offers a free music box (sampler) for traditional and enhanced sounds.
If you’re ok to use 32 Bit Plugins (Might need JBridge), then you can get some cool mixing / mastering devices for free from Variety Of Sound.
Free polyphonic synthesizer plugin from The Wave Warden.
Inch By Inch is a free Max For Live generative step sequencer that creates gradually evolving patterns with probabilities for changing individual steps and other parameters.