talk (Unix)
The Unix command for one-to-one chat in a split terminal, where the other person's words appeared character by character as they typed.
talk shipped with 4.2BSD in 1983 and effectively replaced the older write, which only sent text one way. You typed talk user@host, the talkd daemon rang the other person's terminal, and once they answered the screen split in two. Every keystroke went over the wire immediately — typos, backspaces and the long pause before an awkward reply included.
4.3BSD swapped the protocol for an incompatible ntalk, so two machines on different versions would quietly refuse to connect. ytalk added support for more than two people and spoke both dialects. All of it assumed the other party was logged in, which you sensibly checked with finger first.