CHARGEN
Character Generator Protocol: a service on port 19 that spews characters until someone tells it to stop.
Jon Postel specified it in RFC 864 in 1983 as a test tool. Over TCP the server sends an endless stream of 72-character lines, each one the 95 printable ASCII characters shifted one position from the line before — a slanted pattern that makes dropped or reordered bytes easy to spot. Over UDP it answers each datagram with between 0 and 512 random characters.
A protocol whose entire purpose is to produce lots of data on request is, naturally, a gift to attackers. Paired with an echo port through a single forged packet, it set up an endless loop between two machines, which CERT warned about in 1996. Fifteen years later it came back as a DDoS amplifier, because thousands of printers and elderly routers still had the port open.