it seems like there is a need for some more discussion of the details involved in real-world MongoDB deployments. I thought we’d try and do a couple more posts and walk through some more details about how we’re using MongoDB

Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.

Memcached is an in-memory key-value store for small chunks of arbitrary data (strings, objects) from results of database calls, API calls, or page rendering.

(cdz: featureful alternative to memcached)
Redis is an open source, advanced key-value store. It is often referred to as a data structure server since keys can contain strings, hashes, lists, sets and sorted sets.

CouchDB is a distributed, fault-tolerant and schema-free document-oriented database accessible via a RESTful HTTP/JSON API.

MongoDB (from "humongous") is a scalable, high-performance, open source, document-oriented database.

UnQL means Unstructured Query Language. It's an open query language for JSON, semi-structured and document databases.