Prevayler has caught my interest for a while now as I have been working with Hibernate now for about a year and a half.
Prevayler is a memory resident, file based persistence engine. The objects are kept in memory, and are persisted o disk for permanent storage.
OnJava have an article on their site on using Prevayler. Is it suitable for enterprise systems? No probably not, but looks interesting for small systems or for prototyping simple systems.