CORBA is a popular protocol for writing distributed, object-oriented applications. It's typically used in multi-tier enterprise applications. Recently, it's also been adopted by the Gnome project for interapplication communication.
CORBA is well-supported by many vendors and several free software projects. CORBA works well with Java and C++, and is available for many other languages. CORBA also provides an excellent interface definition language (IDL), allowing you to define readable, object-oriented APIs.
Unfortunately, CORBA is very complex. It has a steep learning curve, requires significant effort to implement, and requires fairly sophisticated clients. It's better-suited to enterprise and desktop applications than it is to distributed web applications.