What MSDN has to say about Extension Methods
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.
A friend of mine started blogging a months ago and he wrote a blog post showing the usage of Extension Methods. I do some of the search and I found a website called ExtensionMethod.NET with a nice collections of extension methods. You can submit your own extension methods to share with other people among the community. ExtensionMethod is a initiative project started by Fons Sonnemans and Loek van den Ouweland.