Building web parts is one of the main extensibility points when you work within a Windows SharePoint Services environment. The development steps for building SharePoint web parts are well-documented in books, in the Windows SharePoint Services SDK and in a whole set of articles, tutorials and walkthroughs available from the SharePoint community or from Microsoft directly itself. One of the major disadvantages of following the traditional web part development steps is that building web parts this way requires extensive coding with respect to the UI you want to provide within the web part. There is no designer available for developers to create the body of a web part in a very WYSIWYG manner.
But there is another approach to building SharePoint web parts. The approach consists of making use of ASP.NET user controls to make up the body of the SharePoint web part. The technique leads to a dramatic increase in the productivity of anybody creating web parts. The idea of using ASP.NET user controls in web part development received a lot of adoption in the SharePoint community and has evolved in a generic framework called the SmartPart.
Read the U2U article Building Web Parts the Smart Way: Starting with the SharePoint SmartPart on MSDN.
Download the SmartPart for SharePoint from the SmartPart for SharePoint: Workspace Home.
This version of the SmartPart will allow you to run ASP.NET 2.0 UserControls and WebParts in SharePoint 2003.
Read more about it here.
You can also download this version from SmartPart for SharePoint: Workspace Home.