WPsyslog is a system logging facility for WordPress. It logs core events like user logins, posts and comments publishing, plugin (de-)activation. It can also be used as in other plugins with great ease. This can be helpful during development or for communicating the plugin's activities to the administrator. Comes with support for Roles and Role Manager.
Link: http://www.zirona.com/software/wpsyslog-wordpress-syslog/