xmsg uses Tk/Tcl and Sun RPC to pop up windows of text to a remote user. It is loosely based on the old cs project xmesg, which required you to munge with your xhost. xmsg instead uses a client-server paradigm to avoid security holes.
Unfortunately, before I could finish xmsg, the cs dept. discovered zephyr (a project at MIT), which is much better than xmsg could ever be. Thus, I never finished the project.
source (gzip’d tarfile).