graphical_force

A plugin that displays the ‘Total Minutes’, ‘Number of Logins’, and ‘Average Time in Minutes Per Login’ that each user has logged on the site. Stats can be exported to csv, and can be reset.

Log Users Stats

Display 'Total Minutes', 'Number of Logins', and 'Average Minutes Per Login' for users with an option to export to csv.

wpsimpletools

Shows default WP / PHP log file from admin interface, gives the possibility to erase it. 
Log viewer is accessible from WP ‘Tools’ menu.

Basic Log Viewer

Does what it says

Useful plugin

Shows default WP / PHP log file from admin interface, gives the possibility to erase it. Only works if debug mode is active.

Log Users Stats vs Basic Log Viewer