I have been spending a little time recently looking at web parts in ASP.NET 2.0. That's right, if you didn't know already the web part infrastructure becomes a fully fledged part of ASP.NET with the 2005 release of Visual Studio. The really great thing about it is the superb developer support - but what more would you expect from the Developer Tools group?
In ASP.NET 2.0 any of the standard web server controls can be used as a web part. Not only that, any user controls or custom server controls you have already written in previous versions of ASP.NET can be used as web parts too! And if you want to make more extensive use of the web part infrastructure you can derive from the WebPart class in the .NET Framework.
If you want to go have a play yourself you'll need the Visual Studio 2005 Beta which you can get from the MSDN Developer Centre.