Hazelcast

A clustering library based on multicast with higher level API than JGroups. Provides shared data structures like maps and queues.
Very lightweight with 1.7MB jar. Runs on rasperry py:)
Static assignment of data keys to cluster partitions. When new node appear, migration of data occurs automatically.
EE edition: elastic memory, jaas security.
Execution of callables can be delegated to nodes.
Persistence support with a map abstraction to implement. Support for write-behind storing data asynchronously in the background.
Many distributed structures supported. AtomicLong allows for clustering a sequence generator.
Plugable behavior to choose between values when nodes rejoin after network partition.