Embed YouTube in ASP.NET
Embed YouTube object into ASP.NET AJAX-extended web page; build simple dynamic playlist control
Silverlight?, YouTube? API (ASP.NET, C#)
Rich Interactive Applications (RIA) built on ASP.NET (2.0/3.5/4.0) platform, utilizing "best of both worlds": embedded YouTube? and Silverlight? Media Players. API, written in C# provides plenty of customization features.