"The Go Text Protocol, GTP, is a text based protocol for communication with computer go programs. It is a modern alternative to the Go Modem Protocol, GMP, and may potentially replace this for use in Go tournaments in the future. It is also intended, through the use of auxiliary programs, to make it easier for go programmers to connect to go servers on the internet and do automatic regression testing."
See http://www.lysator.liu.se/~gunnar/gtp/ for more information.
Clients supporting GTP
Engines (go-playing programs) supporting GTP
GTP version 2 specification will be released soon after GNU Go 3.4.