Authentication Logger is a plugin that allows you to see who logged in by Joomla 2.5 and 3. Provided detailed report and statistics on login.
login

You can read about when a man came here, his username, IP address, and also recorded such data as login failure, password change and other functions.
Advantages
The opportunity to learn who went into Joomla and when.
The drillthrough report.
Simple integration into the available settings and parameters.