MusicSoup

.NET Domain-Driven Design with C#: Problem-Design-Solution
.NET Extensions Methods Library for C# and VB.NET (ASP.NET, MVC, SL, WPF, EF)
.NET Reflector Add-Ins
.NETAsm, a JIT Native Code Injection Library
AJAX Control Toolkit
ASP.NET
ASP.Net Client Dependency Framework
ASP.NET Design Patterns - ASP.NET MVC 2 Case Study Starter Kit
ASP.Net MVC Membership Starter Kit
AtomSite
AutoMapper
AutoPoco
Base Class Libraries
Bundle Transformer - a modular extension for ASP.NET Web Optimization Framework
Combres - WebForm & MVC Client-side Resource Combine Library
CommonLibrary.NET
Community for ASP.Net MVC - http://CommunityForMvc.Net
Community TFS Build Extensions
Continuous LINQ
datajs - JavaScript Library for data-centric web applications