I’ve installed the plug-in for my Network, and activated it for a single site. When logged in as an administrator for that site, I see "Login As..." under the Tools menu. However, the "Log in as" drop-down doesn’t contain any names. Other uses set-up for the site include one other Admin, one Subscriber, one Author and one Contributor.
http://wordpress.org/extend/plugins/admin-login-as-different-user/
