Well, JTD, I was going to explain that John and Jon (short for Jonathan) are two different names and John isn't short for anything, but Wof-doc beat me to it.

John has for a long time been one of the most popular names in Christian countries/families. It takes a good many forms -- John, Jean, Johann, Iohannis, Juan, Giovanni, Johannus, etc.

Short forms of names are also interesting and numerous, and some are a bit improbable. Among my favorites are Stosh for Stanislaus, Gus for Constantinos, Kit for Christopher and there is one for Alexander which slips my mind at the moment.

*post edit: No sooner I posted this than I remembered that Sandy is a short form for Alexander.