BEEP
The IETF's attempt at a general framework for application protocols — so nobody would have to reinvent handshakes and authentication again.
Specified in RFC 3080 (2001) by Marshall Rose. The reasoning was sound: every new application protocol solves the same base problems — framing, multiple concurrent channels over one connection, authentication via SASL, encryption via TLS. BEEP provided that underneath and let the protocol author define only the messages.
It saw almost no adoption. Developers kept building on top of HTTP instead, which was universally available and passed through firewalls. A good example of technical elegance losing to installed infrastructure.