Another interesting(-looking) data store on Java platform: Krati?
Ok, looks like there is one more storage option I really should investigate, Krati. What seems appealing (at first glance) is the understanding that performance optimization on Java platform are quite distinct from those for systems written in C/C++ (or on Erlang and other distinct platforms). And especially trying to make good use of big discrepancy between performance of random access versus sequential access; given that latter can be an order of magnitude faster, it may well make sense to add more processing to be able to sequential writes even if higher-level abstraction was concurrent random-access.
Of course there are lots and lots of other choices: from stripped-down "traditional" storage (like using MySQL InnoDB, see for example g414-inno) to BDB variants, Tokyo Cabinet and Redis. And higher-level systems that use roll-your-own storage (like Cassandra does by default). And this is good, I think; for truly optimal performance one-solution-cant-fit-all -- different storage options are best fits for different system designs.