The amount of green screens in film (also known as Chroma keying) and television really goes unnoticed. I used to think I could spot all the places in a film that there was CG, I guess I was wrong. This video may be dated a little, but just imagine what is now possible with the technology at our disposal. Thanks to Reddit user Nuzen for bringing it to our attention, and Stargate Studios for an excellent video.
Update: Stargate Studies has a new reel, just released and embedded below. While different from their green screen work, still cool. So check it out.