The plumber, in the Plan 9 from Bell Labs and Inferno operating systems, is a mechanism for interprocess communication, somewhat similar to copy and paste.
The plumber is a program which handles all the messaging when programs make plumbing messages. Unlike copy and paste, where the user specifies where the destination of some data should go, the plumber interprets the message on the basis of plumbing rules to determine its destination.
The plumber works transparently on a network, fitting in with the Plan 9 methodology.