Powertone is a very simple-to-use plugin for creating duotones ; I have found it to be easy to use and useful too (spares inks), with fast preview and a good control over the process.
Usefulness :
GUI :
This software is now
part of a suite of plugins offered by CreoScitex.
For info and the demo :
CreoScitex
(It appears that this plugin has been discontinued.)