Mike Whusberg over at Geometros alerted me to their C++/C# solid geometry library for CAD development. They have versions for Mac and Windows, with iOS coming soon.
- sgCore C++/C# Geometry Library for OS X or Windows
- Solidgraph OpenSource CAD System
- rtEngine Ray Tracing Engine
- sgCore Extensions (coming soon): sgCore WebGL, sgCore manufacturing, and sgCore 3DS import