The XBox 360 Widget that I use on this site was fairly straight forward to build thanks to mads and the team for making such an awesome framework for widgets. This post will run you through how to create such a widget.
It is fairly easy to create a widget as all you need to have is an edit.ascx to handle the edit fields and a widget.ascx to handle the display of the actual widget.
Edit: This article + code is horribly out of date. There is a better version of this widget located on biztron.net
More...