There are a number of database systems available which are schemaless, useful for storing large amounts of data which doesn't need to relate to other data, such as logs, pages, documents, and so on. Examples of systems available include MongoDB and CouchDB. Generally, each individual record defines its own structure and fields, allowing such systems to be flexible to the data they are needed to store.
In some cases, where we are working with very large data sets, we can gain a performance benefit by using something like MongoDB to:
Store Drupal fields
Store cached items
Store user sessions
There is a module available which provides integration with MongoDB at http://drupal.org/project/mongodb. More information on MongoDB itself can be found at http://www.mongodb.org/. Unfortunately, it isn't possible to simply install Drupal from the start with MongoDB, we can only use databases such as this to store the information mentioned previously. Installation details for MongoDB can be found...