I use IBM Lotus Domino server software (Notes is the client side piece) as the underlying platform for some healthcare web applications. It would be more accurate to describe Domino as a loosely-structured document database with multi-master replication and a public-key security infrastructure. There is a whole application development layer on top of that, plus additional native features for messaging and workflow. Lotus (and later IBM) then built e-mail, calendar, and groupware applications using those basic building blocks. Those applications ship as part of the product, but you can build your own custom apps that operate on an equal footing. It works extremely well for certain classes of applications.

Usually when someone has a bad opinion of Notes/Domino it's because he used an old version of the e-mail application at some previous company, and it did have a sucky user interface. But the current version is on par with anything else out there.