Just released a free Magento extension designed to prevent admin users from sharing credentials (which wrecks accountability when things go wrong). It prevents an admin account from having more than one active session at a time. If a second user logs in using the same username and password, the first user gets redirected to the login screen with a nice message explaining they’ve been kicked because another user logged in with the same credentials.
It’s just a beta release, not thoroughly tested across different Mage versions (developed on CE 188.8.131.52), but the extension is simple enough it should work fine in most cases. Obviously, as with everything, test before pushing to production.
Thanks go to Jared (http://molotovbliss.com/) for adding modman support 🙂
UPDATE: awesomely enough, this feature is baked-in to Magento 2 🙂