The SmartPart is a generic webpart that can contain an ASP.NET user control. Nothing new you would say, but the SmartPart can give your user control access to the SharePoint object model. So from within the code of the User Control you can do stuff with SharePoint! Another feature is that the user control can expose some properties which the SmartPart picks up, so the user can specify values for these properties in the SharePoint site.
Amanda points out that Tom van de Kerkhof has created a Tree-view WebPart based on this SmartPart. It's very cool; I think I'm going to be able to use it in a SharePoint site that I'm setting up for a local non-profit!