In Plan 9, everything literally is a file. Processes communicate using the 9p protocol which is also the basis of the file system; each process also has its own, local view of the file tree which children can inherit (or not). To do IPC with a server process, simply mount it and begin reading and writing to files in the mounted directory. Oh, and 9p is network-transparent, so you can see remote files and processes as if they were local.

It really is a neat idea, and shows what can be done when you take an idea to its logical extreme. But the interface is not human friendly. Real-world workloads are complicated and require kernels that handle the weird edge cases so you can't reduce a production kernel to a simple idea. And 9p performance lags behind other networked file systems. So it's really more of a prototype and a laboratory for generating ideas to be used in other systems, rather than a replacement for production Unix.