That is, basically fixed forever by convention, large userbase and multiple suppliers?
My suggestions :
In practice, a few Unix classics : SSH, the diff / patch formats, rsync, finger. All used on a grand scale by many parties. Multiple implementations. Multiple pieces of software committed to them. No one really trying to change them.
Email protocols are pretty widely supported and fixed.
Git. It’s notionally owned by Linus Torvalds, but he doesn’t seem to have any commercial interest in extending or breaking it. GitHub showed you can build a great commercial site around it without trying to make proprietary extensions. And I can use the same clients to push and pull from my server running the default Git daemon, from Github, or from rival offerings (I’m pretty sure BitBucket / SourceForge / Google Code now offer Git as an option)
Possibly Jabber / XMPP