The AtheOS Graphical User Interface.
Applications only know about highlevel entities like windows, views, fonts, bitmaps, etc etc.
The AtheOS GUI is a much more integrated part of the OS than in your favourite Unix system. The application server is a central connection point in the system. The application server can receive data from one process and deliver it to another through message ports. This is used for drag and drop between applications and for the global clipboard. Each process lives in it's own address space and generally can't see the memory owned by another process.