Basically, what it says on the tin; I need a JTA implementation useable in a Java SE application, ideally one that doesn't carry too much of a framework burden with it.
I recommend Bitronix. Before using any other transaction manager, I recommend thorough testing. Tests like killing power to various machines during each phase of the transaction. You want transactionality to protect you when failures occur. It is surprising how many transaction managers have failed to correctly implement recovery.
Bitronix does need JNDI, which is typically provided for you in a Java EE container, but they include a simple embedded implementation that will support Bitronix in a Java SE application.