I am currently managing and updating 4 sites running wordpress. Now, that my label has launched, I expect that number to increase.
The following plugins will make your admin life easier:

WP Admin Switcher
This plug helps you switch between the sites you manage via a dropdown

Wordpress Automatic Upgrade Plugin
Ever wanted to update your Wordpress install with a click? Here’s the click.

Coincidentally, both are in the Wordpress Plugin Competition so please rate them.