COM is interesting as it implements interfaces using the C++ vtable layout, which can be done in C. Dynamic COM (DCOM) is used to provide interoperability with Visual Basic.

You can also access .NET/C# objects/interfaces via COM. It has an interface to allow you to get the type metadata but that isn't necessary. This makes it possible to e.g. get the C#/.NET exception stack trace from a C/C++ application.