GemFire (from GemStone Systems) offers mutliple topologies for session caching. These include pure peer-to-peer model or a cache server model with replication that can be used to cache sessions outside of the appserver JVMS and avoide expensive replication. It also supports disk overflow so that passive sessions can be moved to disk with in-memory indexes for optimized access to sessions overflown to disk.
Also, GemFire based architectures are characterized by reduced data latency, enhanced scalability, and increased performance for new application server, portal and integration deployments
More information is accessible at:http://www.gemstone.com/products/gemfire/enterprise.php