Aztica 6DX is a COM-based 3D Engine for windows using DirectX. It features outdoor / indoor rendering, collision detection, portal-based visibility system, particle & effects system, sound, and its own internal GUI. Create 3D apps in VB6, Delphi, VB.NET or C++.
This early release is in OCX form. It contains valuable info if studied carefully. There are some Delphi and VB6 samples included.