Friendly: Easy Schemaless “NoSQL” Data Storage with MySQL in Ruby
Friendly is a new Ruby ORM (a la ActiveRecord) that lets you easily use NoSQL ideas on regular database engines, such as MySQL. Developer James Golick has written a blog post introducing Friendly that goes into detail on how it works - with code examples. Effectively you get schema-less, document-like storage (with indexes!) but based around MySQL.
If you're not familiar with "NoSQL", it's a blanket/branding term (somewhat like AJAX or Web 2.0) that covers non-relational forms of databases that, typically, have no need for SQL. A common subset of NoSQL technologies is the "document-based database," as provided by systems like CouchDB or MongoDB (for which RailsTips author John Nunemaker has evangelized strongly).
An issue many people are having with the various NoSQL systems available is that they're not all "battle hardened" or as familiar to administer than their existing MySQL or Postgres systems. Social aggregator FriendFeed wrote about how they use MySQL to store schema-less data, and Friendly is based on the same techniques.