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

Minor

Simple plugin to secure your WordPress admin interface thanks to IP whitelisting. Only IP addressess on the whitelist (external whitelist supported) have an access into admin interface.
Another possibility how to access WordPress administration is thanks to secret admin link (you still need to insert correct user login and password).
Your IP address is whitelisted and secret admin link generated every time at plugin activation!

Secure Admin IP

Simply restrict login to your WordPress for the specific IP addresses or users with secret link.

multi-login-checker vs Secure Admin IP