> I sometimes wish for a very simple database with the
> following features:
> 
> 1. Distributed as part of Ruby
> 2. Need not store entire db in memory
> 3. No SQL requirement
> 4. No special efficiency requirement
> 5. Available cross-platform
> 6. Database files are readable cross-platform

Jamis Buck's SQLite bindings library is very good, very well
documented, and it is built on top of the SQLite public domain library
(www.sqlite.org)

It obviously doesn't comply with (1) and (3),  but even if one doesn't
like SQL I've found the
api Jamis has built to be quite ruby-esque and intuitive.

The SQLite3/Ruby Manual:
http://docs.jamisbuck.org/read/book/3

Of course if the goal is *not having* any SQL at all, this will never be useful.