Virtues of Document Databases

Lotus Notes is a document oriented database: A document oriented database stores information as documents of related data. All of the data within a document is self contained, and does not rely on data in other documents within the database. This can be quite a shift if you’re used to working with a relational database, where data is broken up in to multiple rows existing in multiple tables, limiting (or eliminating) the duplication of data. Although radically different, the document oriented approach is a very good fit for many applications. For some applications, data integrity is not the primary concern. Such applications can work just fine without the restrictions provided by a relational database, which were designed to preserve data integrity. Instead, giving up these restrictions lets document oriented databases provide functionality that is difficult, if not impossible to provide with a relational database. For example, it is trivial to setup a cluster of document oriented databases, making it easier to deal with certain scalability and fault tolerance issues.
Lotus Notes is not the only one. There is the "son of Notes" - CouchDB (I'm looking only at the database part, not programmability etc.) and others. The description above is shamelessly lifted from a CouchDB article.