tarppa

Write all login attempts where both username and password are supplied to a log file. The logfile is relative 
to error.log and is called ‘login.log’, the format : “TIMESTAMP USERNAME SUCCESS/FAILURE”.
In practice this plugin is a rewritten version of the wp_authenticate_username_password function from user.php and replaces 
it in the login process.

WP Login Logging

Write all login attempts to a logfile by replacing default wp_authenticate_username_password functionality

Jan Štětina

This plugin shows which user has access to which posts under Users administration menu. 
User names are in columns so it is suitable only for smaller counts. Works only in conjunction with User Access Manager and requires PHP 5.3.
It was developed only as helper tool for another plugin which I am going to create in near future and I guess it can be helpful if you are interested how User Access Manager works. Plugin code is (or tries to be) well documented.
This plugin will probably not be updated any further. I provide no guarantees whatsoever.

Post Access Overview

Show who has access to which post in a single table. Works in conjunction with User Access Manager.

WP Login Logging vs Post Access Overview