At Quarter of Six, we have a massive amount of sound libraries, covering almost every instrument known to man. This page is focused on real instruments; for synthesizers, go here. If you want to know specifically what we have, check out our list:

East West / Quantum Leap
Best Service
Reason Refills
ProjectSAM
FXpansion Platinum Samples
Toontrack
Applied Acoustics
Vir2
IK Multimedia
Native Instruments
Apple
Modartt
MOTU
Music Lab