Jason Row post some useful links and information regarding creating Visual Studio.NET Add-ins.

edtFTPnet is the first choice of .NET developers worldwide for
incorporating FTP functionality into their applications.

• Widely used throughout the world in many projects.
• Full source code is provided.
• Passive and active modes are supported (PASV and PORT).
• Resuming of interrupted binary transfers supported.
• Events for monitoring progress of data transfers.
• Under the LGPL, so it can be embedded in commercial applications.
• Mature and stable codebase.
• Simple but extensive API analogous to a command line FTP program.

There are various different ways of implementing the singleton pattern in C#.

The implementation that listed below  is thread-safe, simple, and perform well.

public sealed class Singleton

{

    Singleton()

    {

    }

    public static Singleton Instance

    {

        get

        {

            return Nested.instance;

        }

    }

    class Nested

    {

        // Explicit static constructor to tell C# compiler

        // not to mark type as beforefieldinit

        static Nested()

        {

        }

        internal static readonly Singleton instance = new Singleton();

    }

}

You can read more about different singleton pattern implementations here

If you are interested in some "non-business" development such as games and stuff, you should check out http://msdn.microsoft.com/coding4fun/

"Coding4Fun is all about giving something back to the hobbyist developer community…"  Brian Keller, product manager for Visual Studio

Data Tier Modeler (DTM) is a powerful, ultra-high performance object/relational framework for Microsoft.Net.

New features and improvements (DTM v3.0)

• Improved association classes management
• Improved databinding (MVC model)
• Improved Visual Studio .NET integration
• Rational XDE compliant
• New databases supported : MySQL 4.1, DB2 v8.x
• Optimization of each database specific code
• Optimization of cache mechanism (2 levels cache)
• More documentation and samples

Map many entities

• Object model (UML, XML, XMI)
• 1-1, 1-n, n-1, n-m … relations (aggregations, compositions)
• Inheritance
• Interfaces
• Association classes
• Singletons
• Enumerations

Features

• Objects collections
• Advanced Databinding (MVC model)
• Data encryption (DES or MD5)
• Visitor Pattern 
• XML serialization
• Cascading load/save/delete
• Lazy loading
• Personalized error management
• Read and/or write access

Interfacing

• Fully integrated in Visual Studio .Net
• Code generation directly in your class library in Visual Studio
• UML models management
• Database connexions management (execution of SQL scripts…)
• UML modeling (with Microsoft Visio, Rational Rose, Argo UML…)
• XML modeling 

Request Language 

• OPath (Object Path)
• Object requests
• Bidirectional navigation through objects relations
• Collections for each type of objects
• Criteria
• Sorting
• Paging 
• Template loading
• Visitor Pattern
• Loading of collections, objects or values …
• SQL requests

Database  

• Database specific code (enabling optimization)
• SQL-Server, Oracle, DB2, MySQL, Access, …
• Database concurrency management
• Transactions (automatic or manual)

Unlimited mapping types

• Standard types (int, string, char, bool, DateTime, …)
• Advanced Database Types (xml, text, char[x], decimal[x,y], …)
• Complex serializable types (Hashtable, Drawing.Image…)
• User types
• Binary serialization

Object cache

• Shared memory cache
• Reference tracking
• Creation of Garbage Collection with unreferenced objects.

[ Via Yitzhak Gootvilig's Blog ]