Here are some of the links we've found useful since .NET really got going in early 2001. Please let us know via our contact page if any of the links on this page are broken.

General Discussion / Sample Sites
GotDotNet - Microsoft's community site, with quickstart samples hosted online here.
C# Corner - wide selection of .NET code samples
The Code Project - useful community site with code samples
DNZone - another relatively new community site

CLR
MSDN: Security for Downloaded Code
MSDN: Code Access Security

Threading, the ThreadPool and Asynch Programming
MSDN: Programming the Thread Pool in the .NET Framework - good background info on the workings of the black box that is the Thread Pool
MSDN: Safe, Simple Multithreading in Windows Forms - essential reading for explanation of InvokeRequired etc

Windows Forms
George Shepherd's Windows Forms FAQ - an excellent collection of categorised newsgroup postings etc. with sample code.
GotDotNet: Painting techniques using Windows Forms for the Microsoft .NET Framework

Control / Form Designer Issues
MSDN: Shawn Burke articles on MSDN (Windows Forms / Designer guru)
Google Groups: Shawn Burke posts
MSDN: Drawing Your Own Controls Using GDI+
MSDN: Enhancing Design-Time Support
MSDN: Writing Custom Designers for .NET Components
GotDotNet: .NET Shape Library: A Sample Designer - essential starting point on the control designer learning curve
MSDN: Customizing Code Generation in the .NET Framework Visual Designers
MSDN: Creating Designable Components for Microsoft Visual Studio .NET Designers
MSDN: Make Your Components Really RAD with Visual Studio .NET Property Browser
Creating a struct TypeConverter for PropertyGrid

Reporting and Printing
Crystal Reports within Visual Studio .NET - Licensing Issues
Robert Chapman's evaluation of Crystal Reports.NET
DataDynamics ActiveReports for .NET
MSDN: Printing Reports in Windows Forms
PrintForm .NET - our own Windows Forms printing solution, replacement (and enhancement) of the vb6 PrintForm feature

Control Licensing in .NET
GotDotNet: .NET Licensing Feature Article

Windows Installer (MSI) Projects
Google Groups: David Guyer posts (MSI guru). Windows Installer projects in VS .NET are powerful, and free - but it pays to know their limitations. These posts cover the common problem areas.
installsite.org A list of useful installer tips.

COM Interop
Google Groups: Adam Nathan posts (MS COM Interop guru)
MS Q316581: BUG: Compile Error Message When You Implement a Class in a Referenced COM Library - the commonest problem we found when upgrading vb6 COM servers to .NET was this ByRef parameter to a COM Property issue.

Web Forms / ASP.NET
MS Q322603: HOW TO: Enable ASPX Compression in IIS
MS Q317012: INFO: Process and Request Identity in ASP.NET (ASPNET security settings)
MSDN: ASP.NET WebControls Version 1.0 (Tabstrip, TreeView etc)

Web Services / WSSecurity
MSDN: HTTP Security and ASP.NET Web Services

Mobile
.NET Compact Framework and SDE

Free Tools & Libraries
C# to VB.NET translator - for those times when you're in a VB.NET project and you just want to plug in that C# code...
Chris Sells' Ghengis project - a WinForms library a la MFC
Lutz Roeder's Reflection tools - can show the attributes on classes in an undocumented assembly

Code Obfuscation and Reverse Engineering
Background - Protecting Java Code Via Code Obfuscation
Anakrino - Saurik's MSIL to cs decompiler
Lutz Roeder's Reflection tools - now includes decompiler

Visual Studio .NET
MSDN: Using Visual Studio .NET in a Team Development Environment

.NET Ports and Projects
Runtime - Mono Linux port of .NET CLR and C# compiler (in progress)