The Extension Toolkit project provides a collection of useful extension methods for all kind of types (e.g. String class extensions).
The Microsoft documentation is describing extension methods as follows: 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. For client code written in C# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type.