memcached-session-manager (msm) is a memcached based session-failover and high-availability open-source solution for tomcat, with high-performance, scalability and reliability as it's main goals.
For sticky sessions, an additional copy of a session is stored in memcached for failover, while for regular requests the session is served from the local session map - as fast as usual. Crashes of memcached nodes are handled, sessions are migrated to other memcached nodes in such a case.
For non-sticky sessions, memcached (or whatever memcached compatible, perhaps persistent KV store) is the primary session store, therefore the session is loaded from memcached if accessed by the webapp and stored in memcached (if modified) when the request is finished. To handle the crash of a memcached node for non-sticky sessions each session is stored additionally in another, secondary memcached node.
msm is open-source (Apache License 2), the source code is hosted on github, for discussions there's a mailing list on google groups.