For those who don’t know what is SpagoBI, please read the following (copied from Wikipedia):

SpagoBI is the only entirely Open Source Business Intelligence suite, belonging to the free/open source SpagoWorld initiative, founded and supported by Engineering Group.
SpagoBI supports day-to-day and strategic business, both at the decision-making and operational levels. SpagoBI is a BI suite because it covers the whole range of analytical needs, supporting developers, testers and administrators in their daily activities.

SpagoBI is not very difficult to install but it is a bit more complicated to configure.
One of the problem I had was the following one:

GRAVE: Cannot open connection.
org.eclipse.birt.report.data.oda.jdbc.JDBCException: 
Cannot load JDBC Driver class: com.microsoft.sqlserver.jdbc.SQLServerDriver

This exception is quite clear, it means that SpagoBI cannot load the JDBC driver for SQL Server. Indeed, I was connecting to a SQL Server database from one of my BIRT report.

What you have to do to fix this problem is:

1. Download the driver JAR file from the Microsoft website: http://www.microsoft.com/download/en/details.aspx?id=2505
2. Copy the JAR file into the following folder under the SpagoBI installation folder:
   webapps/SpagoBIBirtReportEngine/WEB-INF/platform/plugins/org.eclipse.birt.report.data.oda.jdbc_2.6.1.v20100909/drivers
   Note that the path might differ a little bit depending on the version of SpagoBI.
3. Restart SpagoBI