JMSJCA is now a java.net project. It can be found here: http://jmsjca.dev.java.net.
The project is currently used in Java CAPS, JMS Grid and the JMS BC as part of the open-jbi-components project.
The connector can be used as a J2EE 1.4 Resource Adapter, but its libraries can also be used as an abstraction layer to JMS servers from non J2EE-code. As such, the adapter acts like a library that hides the complexities of transactions, concurrency, connection failure detection, JMS server implementation idiosyncracies, etc. That is how it is used in the JMS BC as part of the open-jbi-components project.