Single Login is a session management system for Drupal. It allows the site administrator to create a policy to detect, and prevent, duplicate logins on the same account. This is obviously handy for a site that requires paid subscriptions. Once a duplicate login is detected from a different system, the first login gets logged out. The admin can set a policy that determines how often and within what time period a session can "ping pong" between machines. Should the policy conditions be met, the admin can specify an action,typically to block the offending account.
The module also keeps a history of duplicate logins, and if you use the Google Analytics/urchin module, it will insert the session ID.
Your feedback would be most appreciated, and if you write your own plugin, please consider contributing back!