They can be from movies, literature, tv, anywhere. I have always been partial to Batman, Sherlock Holmes, Poroit, Quincy M.E., ****** Dowling aka Mr. Cunnigham, and The Cosby Mysteries (which they should have never canceled!)
I do like Dr. Who being mentioned in the thread, although he was a "doctor", not a detective. A sonic screwdriver is not a stethoscope (although, it might function as one, if the plot line required that).