Finger
The protocol that let anyone ask a machine who was logged in and how long since they last touched the keyboard.
Les Earnest wrote the original tool at Stanford around 1971, after colleagues tired of asking each other who was at which terminal. The networked version was standardised as RFC 742 in 1977 and listens on port 79. Running finger user@host returned a full name, last login, whether mail was unread — plus the contents of the .plan and .project files in the user's home directory.
The .plan file became an early form of microblogging. Through the 1990s John Carmack kept a famous .plan logging the development of Quake day by day. The protocol was equally a gift to attackers: it leaked valid usernames, and the 1988 Morris worm spread partly through a buffer overflow in fingerd. Almost nobody runs it now.