ASP.NET Framework Developer Center - Full of useful articles and information about developing with ASP.NET.
GotDotNet - The .NET Framework community forum.
.NET Framework Developer Center - The home of .NET development.
.NET Homepage - A good starting place for non-development .NET resources.
MSDN Library - Microsoft's premium developer resource brimming with material related to all Microsoft products, with extensive referenece material and articles about the .NET Framework.
GotMono - The Mono community website.
Mono - The home of the Mono project.
Monologue - An assortment of Mono-related blogs.
DotGNU - The home of the PNET's parental project, DotGNU.
getdotgnu - The dotGNU community web portal.
Portable.NET - The home of the Portable.NET project.
MyXaml - The open source XAML implementation.
#WT - A C# port of Java's Standard Widget Toolkit (SWT).
VG.NET - An animated vector graphics library for building .NET interfaces.
prj2make-sharp - Converts Visual Studio .NET proj files to a makefile suitable for use with either mcs or csc.
Apress - The Authors Press and the book's publishers.
Avanade - The leading global integration consultancy specializing in the Microsoft enterprise platform; employers of M.J. Easton.
Brian Ritchie's site - An excellent resource for all things .NET.
ECMA - The European Computer Manufacturers Association.
ISO - The International Standards Organization.
Profox - THe UK's premier consultancy in call centre software, employers of the book's other author, Jason King.