As from today, IBM is releasing its Java-based Cloudscape database to the Apache Software Foundation. This is the first fully functional Java database to be available to open source so far. But more may follow.
Prior to this, the main choice for databases in the open source arena was MySQL, but IBM (who are backing Java, Linux and open source in a number of ways) would much rather have something that they can directly control.
IBM plans to release a commercial version of the product as well.