First Impressions About Xcode Developer Tools 3.0
Dashcode: Very amazing and easy development way to Widgets.
There are a few of templates:
Navigation menu with objects hierarchy
You can add objects with drag and drop.
The Dashcode app is in the Leopard installation DVD, you can also download it form the developer site of apple, with the XCode 3, installation image. You must to register as developer.
You can download a widget to read the rss of appleresearch, was made with a template about in five minutes or less :)