0.5.0 has just been released. You can download it here. This release brings much improved stability, better handling of mapping definitions, and several features:
Several new queries have been added, including
queryString with multiple fields.
terms API allowing to get terms (from one or more indices) of one or more fields and their respective document frequencies (how often they exists in documents). This can be very handy to implement things like tag clouds or simple auto suggest.
cluster_health API for simple indication on the health of the cluster, as well as the ability to wait for the cluster to reach a health status.
moreLikeThis API to search for documents that are like a certain document.
Java API exposing all of elasticsearch operations/actions using simple, transport based, async API to use with any JVM based language.
There are many more minor features and bug fixes, all listed here under the