Prevayler looks like an interesting solution for small scale object persistence in Java.

There are a couple of articles about it over on Advogato.

The big problem with this so far as I'm concerned, is that everything is kept in RAM (like Gadfly). They can talk about Breakthroughs In Memory Technologyand servers with multi-gigabyte RAM, but I'm used to working with multi-gigabyte databases, and in the real world, you just don't have that kind of RAM available.

I'd like to give it a try for something small, though.