Extending Base Type Functionality with Extension Methods

"Extension methods allow a developer to tack on her own methods to an existing class in the .NET Framework. Extension methods make it easy to add functionality to an existing type using a natural syntax. Also, since the extension methods appear in Visual Studio's IntelliSense drop-down list, they are easier to find and use than wrapper class equivalents."

http://www.4guysfromrolla.com/articles/120507-1.aspx

http://dotnetslackers.com/articles/aspnet/5-Helpful-DateTime-Extension-Methods.aspx

http://www.lostechies.com/blogs/joshuaflanagan/archive/2011/02/03/helpful-datetime-extension-methods-for-dealing-with-time-zones.aspx