Design Patterns: Fundamentals

Fundamental design patterns are divided into 3 categories:

• Creational
  • Singleton
  • Builder
  • Prototype
  • Abstract Factory
  • Factory Method
• Behavioral
  • Mediator
  • Visitor
  • Template Method
  • Iterator
  • Command
  • Memento
  • Observer
  • Interpreter
  • Chain of Responsibility
  • State
  • Strategy
• Structural
  • Decorator
  • Proxy
  • Composite
  • Facade
  • Flyweight
  • Bridge
  • Adapter

EDIT: The above patterns are called the 23 Gang of Four (GoF) patterns.  More information can be found from this link.  The site provides a description, a UML class diagram and C# code as well.