Karl Stiefvater runs a course over at CGWorkshops on Creating Maya Plugins, and is sharing a plugin that he developed called qDisplacement for Maya. qDisplacement for Maya is a maya deformer that uses displacement maps to deform geometry right in the viewport.
Check out the qDisplacement Deformer for Maya Displays Displacements Live in the Viewport here.
There is also classic DisplaceD by Hajime Nakamura.
http://www.hajimenakamura.com/hajime/cg/mll/displaceD/displaceD.shtml