Doors are a form of IPC that were created for Solaris. They allow a program to call a function in another program and are, or at least were, much faster than other forms of IPC.
I am submitting the Wikipedia entry because I could not find a better one. The references in the page are mostly dead or not very helpful.
Based purely on reading about Door, I’d love to see them be implemented in more OSs. I think they could make IPC that isn’t through sockets and HTTP a valid alternative for many developers.