Business Data Catalog.
Recently, I had to evaluate and implement the Business Data Catalog(in Sharepoint). I found it to be very useful. The following article by Sahil was very useful during my learning phase.
Getting Started with Business Data Catalog.
The BDC Editor tool available with MOSS SDK was useful in getting me started, but later, I edited the Application Definition XML file manually. This proved to be easier than the editor for me.
