Red Giant Holomatrix is a new After Effects filter/expression package from Dan Ebberts and Aharon Rabinowitz that helps you make your footage look like holograms, ghosts, digital signs, or bad TV. Aharon Rabinowitz has a basic introductory video; the first in a series, [and soon followed with 2 more videos on perspective corner pinning and 3D & the Transform properties].
Also, Alicia VanHeulen of Toolfarm walks you through how to make a ghost effect without the need for additional filters:
Update: check out the "super crazy cheesy" 2-year anniversary episode of the Cow AE Podcast Creating a Hologram Look in After Effects by Aharon Rabinowitz. It's now on Adobe TV: