The Mellotron is really one of, if not the, first sampler and used tape loops. From what I've heard/read it was a real beast to keep working properly, but it definitely left its mark on prog. I guess one of my fav tunes where it is heavily featured is Genesis' Watcher Of The Skies. Here's a little video about the Reason Abbey Road Refill samples in which they open up a Mellotron M400 and discuss it a bit, the Mellotron coverage starts at about 1:10 into the video thru about 3:00.
http://www.youtube.com/watch?v=-sLpo9nPeLk