Clef is a password-less login solution that utilizes a phone app and WordPress plugin for more secure logins. You can get more specific information about two-factor authentication (which is different than this, but along similar lines) on my post where I experimented with two-factor authentication on WordPress.com.
Clef has now partnered with WPMU DEV to offer password-less login integration with all of WPMU DEV’s plugins that utilize logins, such as the Membership plugin.
