Database system that is not relational

Question

What are the other types of database systems out there. I've recently came across couchDB that handles data in a non relational way. It got me thinking about what other models are other people is using.

So, I want to know what other types of data model is out there. (I'm not looking for any specifics, just want to look at how other people are handling data storage, my interest are purely academic)

The ones I already know are:

1. RDBMS (mysql,postgres etc..)
2. Document based approach (couchDB, lotus notes)
3. Key/value pair (BerkeleyDB)

Answer 1

db4o

Quote from the "about" page:

db4o is the open source object database that enables Java and .NET developers to store and retrieve any application object with only one line of code, eliminating the need to predefine or maintain a separate, rigid data model.