Today I ran across CouchDB in Ten reasons why CouchDB is better than MySQL (provides a high level overview but not terribly interesting) and a more interesting discussion among a bunch of database guys (which I'm obviously not,) about what sort of problems this (and similar approaches) are most suited for. I must say after having played around with ORM (ActiveRecord, Django, Hibernate) over the years and more recently had my nose is Moodle databases I'm sympathetic to the idea that maybe not everything should be stored in tables, rows, and fields and having to design (or discern) the relations. There is just something sort of contorted about the process of viewing the world (or the data we are trying to capture in the world) this way. I have also definitely felt the pain of having to adjust your schema (as you realize new requirements) and perform "migrations" so there is definitely something intriguing about CouchDB.
Some of the aspects I found interesting
In an SQL database, as needs evolve the schema and storage of the existing data must be updated. This often causes problems as new needs arise that simply weren’t anticipated in the initial database designs, and makes distributed “upgrades” a problem for every host that needs to go through a schema update.
CouchDB is a peer based distributed database system. Any number of CouchDB hosts (servers and offline-clients) can have independent “replica copies” of the same database, where applications have full database interactivity (query, add, edit, delete). When back online or on a schedule, database changes are replicated bi-directionally.
And there is also OReilly book in progress that provides a much more readable introduction and Standalone Applications with CouchDB is also definitely worth reading.
(Oh and on Ubuntu 8.10 it is in the repo so it is an "apt"-get away)