I keep hoping that the creators of fallabs make a triumphant return. They did post a small bugfix for a build error not too long ago, so the lights aren't completely out.

We used kyotocabinet exclusively for a project that ran for 3 years with ~1 billion txns/day in a big AWS cluster. Totally bomb-proof and a great piece of software. There's no reason not to use it today, really. Tycoon, the server, is good, but the library kyotocabinet is multithreaded and highly performant.

The flexibility of B+ tree or hash type stores, on-disk or in-memory with the same interface is extremely useful for building distributed systems. Very stable under load, and no data bloat as records pile up.

edit: nobody should feel guilty for using stable software with a reliable track record. Just because a project isn't being printed on tee-shirts anymore and breathlessly fawned over during the free happy hour at conferences doesn't mean that it isn't still a perfectly useful piece of software.