Web Parts: From SharePoint to ASP.NET 2.0 by Dino Espositio
http://www.theserverside.net/articles/showarticle.tss?id=WebParts1
This article demonstrates how to develop a web part for SharePoint Server, which calls an Amazon web service. The source code of the web part is very simple but very instructive as well.
The source code of the web part is here if you are interested in implementing web part for SPS 2003.
http://www.theserverside.net/articles/content/WebParts1/Source.zip