Game Middleware

Audio

FMOD by Firelight Technologies
Wwise by Audiokinetic

Mesh Optimization

Simplygon by Simplygon Studios (Microsoft)

Networking

Photon by Exit Games
PlayFab by Microsoft
RakNet by Oculus (Facebook)
SpatialOS by Improbable

Physics

Bullet by Erwin Coumans et al.
Euphoria by NaturalMotion
Havok by Havok (Microsoft)
Newton Dynamics by Julio Jerez et al.
Open Dynamics Engine (ODE)
PhysX by NVIDIA

User Interface

Coherent Gameface by Coherent Labs
NoesisGUI by Noesis Technologies
Ultralight by Ultralight

Vegetation

SpeedTree by Interactive Data Visualization (Unity)

VFX

PopcornFX by Persistant Studios