I think Lotus Notes has a fascinating history, cryptographic shenanigans asides. As far as I'm aware it is still the most successful "offline first" / "local-first" application platform in widespread use today, complete with multi-master document replication, highly-configurable conflict resolution and rapid application development. These were revolutionary capabilities during the dialup era. The email client just happens to be the most popularly recognised groupware application outside of individual corporate firewalls.
The original vision was very lofty: https://web.archive.org/web/20180225100127/http://www.kapor....
"Notes should take the first major crack at the area of idea processors, textual databases, and hypertext systems."
I would love to see where an open source equivalent built for the modern age could take us... (CouchDB gave it a good shot!)