I just read a really interesting whitepaper about an object database for Java called GemStone Facets. It’s basically like virtual memory for persistent Java objects; objects are faulted into local memory from a shared transactional cache. The downside is that it requires a custom JVM, currently based on the Sun JDK 1.4.2 codebase. Supposedly, they’ve released a free Linux version, but the license does not appear to be easily accessible. (The download is a 111MB install script.) Still, it’s an interesting idea.