Open Graphics Engine (Ogre). High-level adaptor and extension of DirectX technology.
Open Design Alliance SDK (on demand). Support for .dwg and other AutoCAD formats.
OCCT and Ogre are open-source components. This means they are modifiable and extendable. With or without modifications, there is a way to access them and pass information and execution both ways from/to the application as demonstrated in v5.2 Bridge sample (coming back soon in v7).
DG Kernel provides high-level interface, which automates and simplifies lots of complicated functionality.