Bar none the best CoC adventure is this one.
http://www.amazon.com/Pursuit-Kadath-Call-Cthulhu-Gallagher/dp/B0006YMOI6/ref=sr_1_1?ie=UTF8&s=books&qid=1228003038&sr=8-1
it's rare, (as the price should indicate to you) and anyone that has it knows why it's worth every penny.
The plot starts out like this.
The players are all hanging out in the school library together. They all either attend, teach, or in some way work for the college they're all at.
They've all been friends or at least associates for a while and one of their number comes running in and shows them a newspaper.
In it, they see a picture and a headline. Something on the order of "floating arm seen in alley after bank robbery". In the picture they see a friend of theirs, Niles Lindstrom holding the bloody arm that used to be attached to a bank security guard. The guard is on the ground, holding the gaping wound where his arm used to be. In Niles other hand is a large black leather satchel.
But here's the kicker... noone else can see him in the picture. They show it to other people, and they completely fail to see Niles.
The spoiler to this whole thing is that some nights before this, all the party members, along with Niles, went to a party at the home of a fairly famous archeologist fellow at the uni. At the party they were hypnotized (a popular party trick in the 1920s), made to perform tricks, relive past lives, then made to forget the whole thing
