I wanted to know when new accounts were being created on my blog software (Nucleus), and when people were logging in to it. As this wasn't available in the core Nucleus functionality, I've written a plugin, called NP_Paranoia, to do this. Very roughly, it sends an email when a new account is created, when an account is deleted, when a successful login occurs, and when a failed login occurs. Each of these can be turned on and off as required, and they can also be recorded along with the requesting IP address in the Action Log.

Let me know if you use it and it's any use, and if there's any other features you would like, I might be able to accomodate them as well.