This caught my eye as their performance stats compared to other traditional RDBMS are staggering.
Prevayler is an in-memory Object persistence mechanism as a direct replacement for a database. The in-memory representation of your objects is the database, so their is no need to query the objects to retrieve them. This also means you can navigate object graphs as you normally would, without using any complex query language.
So how does it keep data persistent? Apparently it serializes all your object out to disk periodically.
Incredibly simple, but I bet incredibly effective. If you don’t have a need for GB of relational data, the this seems like an ideal solution for small webapps etc.