NHibernate (.NET O/R mapping tool) is still in Alpha, but it seems that beta will appear soon. I have a quick look at this Java clone. Well, this tool may be an alternative to NEO (another open source O/R mapping tool).
NHibernate lack of code generation for now, but it will be added in further versions. XML mapping file are more clear in NHibernate, but in general NEO and NHibernate have very similar concepts in their cores. I think NHibernate will be more popular and grow up faster, since there are many contributors behind. But for now NEO is better.
P.S. Good NHibernate intro article by Justin Gehtland.