What is your most commonly used namespace in .NET. I know it will vary greatly based upon the types of projects you develop but the stack overflow audience should provide a fairly decent sample set for the types of .NET projects being developed.

I'm simply interest in the name of the namespace (one namespace per answer and no one person should have more than one answer, if someone else has the same answer as you then just upvote their answer). Try to be as specific as possible (so answering System, isn't helpful).

I'm after this information to help new developers focus their attention on the most common .NET namespaces...there are after all thousands of them!

To start off mine is almost certainly System.Collections.Generic, I use lists of things everywhere.