A Plugin That only allows whitelisted IP's, or optionally whitelisted browsers, to access wp-login, or optionally wp-admin.
This plugin also does Not effect front-end login plugins.
The Whitelist plugin enables you to open your website to a set of users and blocks everyone else. You can enter users by their email address into the Whitelist tab of the plugin. Once done, only users on the list would be able to visit your site.