There are two leading open source ETL software:
Talend Open Studio
Provided as a packaged, out-of-the-box, ready-to-install platform, Talend Open Studio is one of the most used open source ETL tool. Please note that this is an enterprise supported open source software, the community of users is pretty large but there is not a large community of contributors.
Pentaho Kettle is a set of open source ETL tools that will all you to manipulate data from various databases. It gives a graphical user environment to describe what you want to do not and how you want to do it. As for Talend, please note that this is an enterprise supported open source software, the community of users is pretty large but there is not a large community of contributors.
Other open source ETL software worth mentioning:
CloverETL is an open source,Java-based, ETL framework which can be used to transform structured data. It can be used standalone as an application run from command line or can be embedded into an application.
Apatar is an open source data integration and ETL tool written in Java, with powerful Extract, Transform and Load capabilities, that enables anyone to join their on-premise data sources with the Web without coding.
Enhydra Octopus is an advanced relational Extraction/Transformation/Loading tool. It can connect to JDBC data sources and perform transformations defined in XML definitions. JDBC drivers for CSV and XML are included. Octopus supports Ant and JUnit.