This isn't my plugin just found it on my computer and I see it isn't released on the MyBB mod site anywhere and the people who made it isn't around anymore but I don't take credit for this plugin it belongs to the "MyBB Security Group" at http://www.mybbsecurity.net/.

Plugin Name: Mod CP Password
Plugin Desc: Requires a password to enter the Mod CP.

Tested and Working:
 - MyBB 1.8
 - MyBB 1.6