Loginner gives you automatic access to other WordPress backends you own, in one click.
Every account is put in the admin bar for easy switching.
Easy customization of active accounts and user rights.
WARNING: THIS PLUGIN CAN BE INSECURE IF NOT USED CAUTIOUSLY. USE IT AT YOUR RISK!
For more informations: http://www.duckinformatica.it/prodotti-e-servizi/free-wordpress-plugins/