storyday

force user login into wordpress only from one browser at the same time 
the last login action will kick out the already logged user 
this is a security plugin for wordpress,install this plugin to avoid simultaneously login

multi-login-checker

force user login into wordpress only from one browser at the same time

alvego

Simple and easy WordPress plugin for protecting your login form.
Plugin implements setting the security key for the login form.
Thereby significantly reducing the likelihood of automatic brute force.
The security key is transmitted manually as GET parameter for login form.
For example: http://example.com/wp-login.php?sk=my_secure_key
Warning: Be careful if <code>sk</code> parameter is not specified, the login will not be possible, even if specifying the correct username and password.
<h3>Arbitrary section 1</h3>

Protected wp-login

This plugin protects your admin's login page (secure key in url).

multi-login-checker vs Protected wp-login