The London thread got me thinking about this...Disneyland uses many different effects, ranging from the very low tech and probably low cost (such as the clear plastic "clouds" found in the London scene of Peter Pan), to the high tech and pricey (the Davy Jones projection in POTC). It's clear that the imagineers of Disney are nothing short of brilliant, IMO.
So what's your favorite effect?
Mine has to be the Pepper's Ghost in the HM Ballroom scene. I love to watch the "ghosts" as they appear and disappear.