I think that Talend Open Studio is the best open source ETL software out there. I have built a Open Studio component called tBirtOutputExec that automatically outputs ETL processes to BIRT reports.
The tBirtOutputExec component is compatible with version 2.4 (and above) of Talend Open Studio. You will need to download Open Studio from http://www.talend.com/download.php and to get the BIRT component from the Talend Ecosystem: http://www.talendforge.org/ext/extension_view.php?eid=57. There are also some pretty well designed tutorials on how to use Talend: http://www.talendforge.org/tutorials/menu.php.
See also this post in Talend's forum: http://www.talendforge.org/forum/viewtopic.php?id=3246