SIMPLE [1], mostly. There is also IMPP [2], an XMPP competitor which has a bunch of RFCs behind it, but never gained any implementations, as far as I know.
Alternatives such as the MSN, AOL and Skype's IM protocols are shoddily defined, cruft-ridden ad-hoc protocols that people have been mostly able to reverse engineer, but they are not useful except for interoperability with existing networks.
SIMPLE has not been deployed to any great extant, that I know. The benefit to XMPP is that it's not just well defined (a bunch of RFCs with a working group behind them), but also a proven protocol that has years of production use under its belt. The problem with the IM protocol landscape is entirely political. Everyone wants to own the network, nobody wants to be compatible.
Alternatives such as the MSN, AOL and Skype's IM protocols are shoddily defined, cruft-ridden ad-hoc protocols that people have been mostly able to reverse engineer, but they are not useful except for interoperability with existing networks.
SIMPLE has not been deployed to any great extant, that I know. The benefit to XMPP is that it's not just well defined (a bunch of RFCs with a working group behind them), but also a proven protocol that has years of production use under its belt. The problem with the IM protocol landscape is entirely political. Everyone wants to own the network, nobody wants to be compatible.
[1] https://en.wikipedia.org/wiki/SIMPLE
[2] https://en.wikipedia.org/wiki/Instant_Messaging_and_Presence...