My experience with the term "add-in" was that you have a piece of software put on top of a bigger software, just to enable some little functionality. Today at SQL Pass 2006 I've seen how this term has to be redefined.
The Data Mining Add-ins for Office 2007 (CTP Nov 06 downloadable here) transforms your Excel 2007 and Visio 2007 products in full featured Data Mining clients. It requires Analysis Services 2005, but no work is required, the client can be used both to browse existing models or to create whole new models, starting from data contained in Excel or accessible from Data Sources defined into Analysis Services 2005 databases. One limitation of the current CTP (Nov. 06) is that it requires the Beta 2 Technical Refresh version of Excel 2007; a new add-in will be released shortly with a full support for Office 2007 RTM.
This "add-in" changes the accessibility of the Data Mining features of Analysis Services 2005. Yesterday Microsoft only had the server, today Microsoft has also the client, that fully leverages the power of Excel 2007. Guys, the demo was really impressive for the easy of use and the power that this add-in gives to end-user, not to mathematicians.