- Atomikos TransactionsEssentials™
- JBOSS Transactions, which since 3-April-2006 includes technology aquired in 2005 from Arjuna.
- JOTM, the transaction manager developed by ObjectWeb.
- The Tyrex Transaction Manager.
- Simple-jta - A standalone JTA compliant Transaction Manager
- Bitronix Transaction Manager - a simple but complete implementation of the JTA API.
- See also here for some details on Glassfish's transaction manager.
All of these are usable/configurable outside a full blown J2EE container (I'm not sure about JBOSS Transactions though, anyway it's highly probable that it can be done).
All of these are supported and can be easily integrated with the Spring framework, if you don't want to use a J2EE container.
For more on Spring, Jencks and Geronimo Transaction Manager see here.
For more on Spring and JOTM see here.
For more on Spring and Atomikos see here.
For more on Spring and JBOSS see here.
For more on Spring Transaction Abstraction see here. It's not related to JTA entirely but it will give you some usefull info about transactions in general.
Thanks to Guy for replying and pointing me where I was wrong about Jencks.