You can put this widget in the menu or header widget. It will create Testusers for each available role in the application and allows you to switch users with the combobox provided.
There is a siteproperty to turn this feature off. Default is turned off because of the impact it can have on Production environment.
To be honest: this component was the idea of a former colleague (you know who you are!). I have recreated it because it is very handy when you have an application that has many roles you need to test.