Open Source Business Intelligence/DWH solutions I wonder that this question hasn't already been asked. Google only has very few results for me that don't show a high quality tool

What are some Open Source (also free is ok) solutions for Data Warehouses and more specifically Business Intelligence Tools? What are your experiences with them. I have had a course in my masters programm and we worked with MS Business Intelligence and MSSQL as Data Warehouse storage. Now I want to get more into this topic with tools that are "open".

Are there any compareable tools for Business Intelligence (mostly database independent) and do you have any experience with them?

edit With Marian's comment to Stephanie's answer, I see that I have formulated the question wrong. I am aware that DWH are just "reporting-optimized" databases, Stephanie's explanation was very clear on that. I am more interessted how to get the data into such optimised forms with WHAT kind of BI software/tools/other techniques.
I have been working with Pentaho for about a year now.

Pentaho is a full Open Source suite for Business Intelligence.

It's strenght is that it relies on independently managed project :


Pentaho Data Integration (Kettle) ->ETL 
Pentaho Report Designer (PRD) -> Report designer 
Mondrian -> R-OLAP cube
and much more..


You can use them as a whole (Pentaho BI Server) or apart.

Why would I recommend it ? The community is really active and provide plugins on a weekly basis. A group of community members provided a framework for building interactive dashboard with a click&amp;go ui.

It is database independant. If you have some free time just take a look at it.

I did a research before using Pentaho, I did test IceCube and Palo even JasperSoft but Pentaho was from far better according to my needs.