The only "ghost effect" product I know of is Fallen Angels, Restless Wraiths, by cyllan, which is no longer available here.

And the thread Acadia linked to is a good way to do ghosts, unless you want parts of them fading away, which would require layering renders, but would otherwise be easy.

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If you're joking that's just cruel, but if you're being sarcastic, that's even worse.