Business intelligence covers a wide area, e.g. tools to help build reports, pre-packaged solutions that work with specific apps, etc. I would start here and check out some of the free and open source solutions.
I don't know what features are you looking for, but I really like Pentaho. The only downside is sometimes you have to hack the code to get the features you want since the source is provided for you aswell. But all in all I think it is a very professional BI tool.