Spitfire LABS is the free collection of Spitfire Audio.
It contains a whole slew of instruments and effects. From normal orchestral instruments to foghorns and city sounds.
VSTi & VST3i.
8dio has a bunch of free instruments and synths available.
Also a great sampler if you want to make your own virtual instrument.
VST.
VSL has besides its Big Bang Orchestra some individual instruments available.
Examples are Strings, harps, piano.
VST.
Pianobook is a project supported by Spitfire Audio for individuals to post their own VST's
All kinds of instruments and sounds are available.
Using Decent Sampler or Kontakt or Sfz.
Beware for Kontakt you need the full Kontakt player, not the free one.
SINE Factory is Orchestral Tools' free collection of instruments.
VST.
Native Instruments has stuff of about everything for free:
The free version of Kontakt Player.
Komplete Start, a minicollection of things.
some instruments, synths and effects.
I've been using this for synth things so it deserves a place here.
VST.
Has a diverse range of things available.
Effects, instruments, synths,....
Usually VST, check to make sure.
I've been using Reaper for over 15 years and it includes Reaplugs.
You can download reaplugs seperately for free and it has the basic effects you need for producing music.
Some free fun instruments.
VST (Free Kontakt Player)
Synth and beats for you hippedyhoppedy and trippytrappy music.