Hello,

Best free VST Plugins For Mastering are :

Span - PC / Mac

Loudmax - PC / Mac

Deespeaker - windows / Mac

Limiter 6 - PC /Mac

MV meter - PC / Mac

FerricTDS - PC / Mac

Stereo tool V3 - PC / Mac

A1 Stereo Control - PC / Mac

After-windows

Bus compressor - windows

Marvel GEQ - PC / Mac

Softclipper - PC / Mac

Dpmeter - windows/ Mac

Hpl2 - windows / Mac

Klipfreak - windows / Mac

MHORSE P3 Mkll - windows

Hope it helps