Massive Free instrument and effect audio plugins database for Windows, MacOS and Linux.
KVR Audio is an excellent pillar in the music community and also holds developer contests every year for audio plug-in/audio application/soundware design creators.
Izotope is currently offering 3 quality free virtual instruments: Vinyl, Ozone Imager, and Vocal Doubler. Check them out while they’re free.
Kassee currently uses Ozone Elements for mastering tracks.