ZMODEM

ZMODEM was designed in 1986 by Chuck Forsberg and became the dominant way to move files over a BBS connection. Where its predecessors XMODEM and YMODEM stopped and waited for an acknowledgement after every block, ZMODEM streamed data continuously and confirmed after the fact — far more efficient on lines with latency.

Two features stuck in particular. Crash recovery let an interrupted transfer resume where it stopped instead of starting over — invaluable when a call might drop after twenty minutes. And auto-start: if the remote end sent the sz command, the terminal's rz woke up automatically and the download simply began. Combined with CRC-32 error checking, that made ZMODEM the standard for warez trading and disk magazines right up until the Internet took over.