The STM implementation using core Narayana features. Documentation can be found at http://narayana.io/docs/project/index.html#d0e16062
You can also find related blog entries in the Narayana team blog at http://jbossts.blogspot.co.uk and we've included a few here ...
http://jbossts.blogspot.co.uk/2009/12/stm-for-arjuna.html
http://jbossts.blogspot.co.uk/2011/06/stm-arjuna.html
http://jbossts.blogspot.co.uk/2012/02/optimistic-stm.html
http://jbossts.blogspot.co.uk/2012/03/update-to-stm-api.html
Note, STM is also the basis of the Vert.x transaction work, which is currently residing https://github.com/nmcl/TransactionalVert.x as well as some of the extended transaction work within WildFly Swarm (https://github.com/wildfly-swarm/wildfly-swarm-examples/tree/master/transactions/transactions-shrinkwrap)