After the YAML and the Dependency Injection, it's time for the Symfony Event Dispatcher component to join the crowd of the Symfony components. This component is not new. It has been introduced in symfony 1.1 and is heavily used in the symfony core to make symfony more decoupled and more flexible.
The Symfony Event Dispatcher component is represented by an Octopus.
Of course, you can start using it right away in your non-symfony projects by reading the dedicated book: