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 away to access them and pass information and excution both ways from/to the application as demonstrated in v5.2 Bridge sample (coming back soon in v6).
DG Kernel provides high-level interface, which automates and simplifies lots of complicated functionality.