Design patterns are recurring solutions to software design problems you find again and again in real-world application development. Patterns are about design and interaction of objects, as well as providing a communication platform concerning elegant, reusable solutions to commonly encountered programming challenges.

http://www.dofactory.com/Patterns/Patterns.aspx

codebetter.com:

Template Method
Strategy
Factory Method
Abstract Factory
Facade
Command

ref:
http://codebetter.com/blogs/jeremy.miller/archive/2006/04/11/142665.aspx

A singleton pattern is one that states that there would be only one instance of a class throughout the application domain. This article illustrates this pattern with lucid code examples.

http://aspalliance.com/810
http://www.dofactory.com/Patterns/PatternSingleton.aspx
http://www.yoda.arachsys.com/csharp/singleton.html