Oplop is actually an acronym for "One Password [equals] Lots Of Passwords". Look at the project website: http://code.google.com/p/oplop/ to learn more.
I needed an npm package, so I copy & pasted one of the available browser based implementations.
$ npm install oplop
var uniquePassword = require('oplop') (nickname, masterPassword)
The code was actually taken from a web implementation by Joshua Dick http://joshdick.net, which was taken from the official Oplop implementation at http://oplop.appspot.com.
The only difference is that this module is using the native Node.JS md5 implementation.