Resources

Windows Phone

• Theme Colors
• Pre-defined system styles and resources in Windows Phone
• Windows Phone Power Tools
• The Windows Phone Toolkit - NuGet: Install-Package WPtoolkit
• Coding4Fun Toolkit
• Complete - NuGet: Install-Package Coding4Fun.Toolkit.Complete
• Controls - NuGet: Install-Package Coding4Fun.Toolkit.Controls
• Audio - NuGet: Install-Package Coding4Fun.Toolkit.Audio
• Network - NuGet: Install-Package Coding4Fun.Toolkit.Net
• Storage - NuGet: Install-Package Coding4Fun.Toolkit.Storage
• Projecting your Windows Phone screen to a PC (Windows Phone 8.1)

Windows Store

• Choosing your app images
• System brushes
• Laying out an app page
• Guidelines for fonts
• Index of UX guidelines for Windows Store apps
• Symbol enumeration

MVVM (Model-View-ViewModel)

• Leveraging Windows 8 Features with MVVM - demonstrates how to write testable applications with MVVM; sharing, settings, pinning, ... The sample code also uses the 'Charmed' library.
• Charmed - a cross-platform MVVM support library for Windows 8 and Windows Phone 8. Enhances your favorite MVVM library in a more testable way. 

Libraries, frameworks, ...

• AutoFixture - write maintainable unit tests, faster.
• AutoMapper - get rid of code that maps one object to another.
• Rhino mocks - dynamic mock object framework for the .Net platform.
• Moq - "The simplest mocking library for .NET and Silverlight".

Artwork

• Clker - not great but free.
• IconArchive (Commercial Free) - different license types. 
• ICONFINDER - different license types.
• UnLucky Studio - Free Game Art every month (free to use in personal and commercial projects - even without credits)

Sounds

• SoundBible.com (Royalty Free)
• freesound

Cross-platform

• Xamarin - Mobil App Development (C#)
• Marmalade - C++ SDK (Lua, HTML or Objective-C)