” . . . the SmartPart is an open source web part that can host any ASP.NET Web User Control.”
“The SmartPart allows you to create a Web User Control (ASCX) in Visual Studio by making use of the designer, and deploy it to a SharePoint site. This release of the SmartPart is targetted for SharePoint 2007, both Windows SharePoint Services v3 (WSSv3) and Microsoft Office SharePoint Server 2007 (MOSS).”
You can read this announcement here.