I've just attended a practical session about the RTM of VS2005 Extensions for WSS3. This Visual Studio Add-in allows to define Web Part, Site Definition, List Definition and SharePoint solutions, easily and quickly working within Visual Studio 2005. Sounds interesting the SharePoint Solution Generator utility, useful to convert a concrete site definition, defined using SharePoint Designer, into a Site Definition to deploy many times. Here you can download it.
Pay attention: the product works only with WSS3 installed on the same machine.