Lot Us Group Ware server created by Ray Ozzie.
Like Ms Exchange, often used just as an EMail system, which is particularly a shame because the Notes-client is so user-hostile.
WikiWikiWeb:LotusNotes
We used it at Med Scape in the late-90s (running on ancient Mac OS). A big step backwards from Now Software.
I tried out a number of Project Management apps built on it (Plat Form), and they all felt like clunky macros, like homegrown DBase programs from the late 80s.