Turings original paper on artificial intelligence proposing the Turing Test that I talked about below is of course available on the net, as well it should be. I found it using the most intelligent piece of software I know, Google.
As remarked by Douglas Hofstadter in the introduction to G?del,Escher,Bach (maybe it's an addendum for the anniversary addition I have; I certainly don't recall it from first reading) it is impressive how well Turing manages to address practically all the objections to the test that have since been given, in the very first presentation of the test. Since Turing is also quite a good writer, the piece is certainly recommended.