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In the opensource world, two integration tools distinguish themselves: Pentaho and Talend.
kettle, a component of pentaho, a complete BI suite.
Talend is developed by a french company. It was awarded "company-to-watch" by the Intelligent Enterprise Magazine.
Integration tools are also called ETL for "Extract, Transform and Load" :
Extract: they read many data sources
Transform: they can apply treatments to data, convert them between different formats
Load: they include "write" features
The advantage of kettle is that it's part of a complete BI suite. The other modules of pentaho are Mondrian, an OLAP server, Pentaho reports, Pentaho DashBoards, Pentaho Weka for data mining.