We have a new release of our popular Master User plugin. This plugin for Joomla allows administrator users to log into the front end of their site as another registered user, and is compatible with Joomla 2.5.and 3.
The new release, version 2.3.0, includes some improved error handling. The interesting thing is that by popular demand we have added a feature to set a session variable when a user logs in as a master user, so that other extensions can pick this up. You can read more on the download page.