Prevaler is a great alternative to using a relational database if you need persistence and your application's data easily fits in memory. I wrote a large web app (similar to SharePoint) about 6 years ago using Prevaler and at one point it ran almost three years without restarting (until the server that it was running on needed maintenance). Prevayler is solid stuff.
Space4J works on the same concept but takes advantage of the Java 1.6 concurrency APIs. Space4J is a new project and I have only had time to look through the source code and examples, but it definitely looks like a possible substitute for Prevayler on future projects. I think that Space4J might perhaps take more advantage of generics (e.g., the interface Space and implementing classes could be "generic-ized" for type safety and elimination of unchecked exceptions).