Future in implementation of unit tests and product - Dependency injection

What is dependency injection? Wikipedia Dependency Injection, Interested link-pdf about Dependency Injection
Why we can or should use it?
Because:
1 We can have better architecture in unit tests.
2 If we have few implementations of the same functionality than we can use Dependency Injection to set few configurations to have better architecture.
3 Client could use our one product in few configurations/functionalities in real time dynamicly.
4 We have opportunities with Dependency Injection to develop next design patterns in our project.
5 See point 1 because it makes sense and gives you big benefit.

Which Dependency Injection libraries we can use in .Net?
1 Unity
2 Castle Windsor
3 Autofac

I'm starting to get knowledge about this, so if you have comments, please add it.