Good introduction to MongoDB at Linux Magazine. It makes the case for post-relational web application architectures quite well:

Web applications and traditional relational databases are nearing an end to their tumultuous relationship. For over a decade now, most Web applications have been built on top of relational databases, with various layers of indirection to simplify coding and boost the productivity of developers. For every Web programming language, there are any number of object-relational mapping (ORM) choices, each with pros and cons, yet none good enough that a developer can forget about SQL or ignore protecting the database. Moreover, as Web applications grow more complicated and sites need to be created faster, adapt instantly, and scale massively, these old solutions are no longer satisfying the demands of the Web.

There are a number of different projects working on new database technologies, all of which forego the stalwart relational model. Relational databases are difficult to scale, largely because distributed joins are difficult to perform efficiently. Further, mapping from the many popular dynamically-typed languages to SQL is complicated, inefficient, and time consuming. While often called the “NoSQL” movement, the need for new technologies is caused by the relational model, rather than SQL.

Beyond the relational model, there are a number of data model choices: key-value stores, tabular databases, graph databases, and document databases…

InfoGrid is built around a graph database in this terminology. Except of course, that by virtue of the Store abstraction, InfoGrid can delegate to virtually any kind of database. InfoGrid does not contain a database itself. We believe this separation of concerns makes it easiest for developers to benefit from the high-level services that InfoGrid offers, while still being able to choose whatever storage technology they prefer.