Luka Petrovic

Enables you to login from trusted IP without entering password by specifying ?bypass_login=username parameter on site.

IP Login

Enables you to login from trusted IP without entering password  by specifying ?bypass_login=username parameter on site.

duckinformatica

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/
<h3>Upcoming features</h3>
<ul>
<li>Support other CMS: Joomla!, Drupal, etc.</li>
</ul>

Loginner

Loginner lets you open other Wordpress backends under your control in one click. WARNING: THIS PLUGIN CAN BE INSECURE IF NOT USED CAUTIOUSLY.

IP Login vs Loginner