I’ve talked to many developers who are excited about Metro app development, but say that the hardest part is coming up with an app idea.  Some folks want to walk through converting an existing app to Metro as a learning exercise.  There are other folks who have very successful websites or apps that they would like to make available on Windows 8 too. 

There are a lot of useful resources out there on porting existing applications to Metro apps that will run beautifully on Windows 8:

Design-focused articles

• Porting an iPad app to a Metro app – this article is focused on design and shows how to transition between these user experiences
• Rethinking a website as a Metro app – this article is focused on design and shows the process of reimagining a food truck website as a Metro app

Porting a website

• Migrating a web app to Metro app – this short article is focused on development and conversion of a front-end web app to a Metro app using HTML/JavaScript
• Migrating a website to Metro app – this short article is focused on development and discusses some back-end considerations like communication, streaming, security, client package deployment, data sharing, and syndication

Porting existing XAML apps (WPF, Silverlight, and Windows Phone apps)

• Migrating a Windows Phone 7 app to XAML Metro app – this article looks at porting a WP7 app written in Silverlight to a Metro app written in XAML.  This includes some helpful info, like a mapping of the Silverlight and Windows Phone namespaces and their equivalents in WinRT, and a list of differences in the UI capabilities. 
• Migrating a Windows Phone 7 app to JavaScript Metro app – this article looks at porting a WP7 app written in Silverlight to a Metro app written in HTML/JavaScript. 
• Porting Silverlight or WPF XAML/code to Metro app – this article examines the major differences between Metro apps and other XAML apps, and gives some advice on converting projects. 

Leave a comment if you find other useful resources on transitioning to Metro.