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Here's a 2 liter plastic soda bottle leyden jar, can be shocking, even dangerous, don't do this unless you know what you're doing (here's what they're all about, Wikipedia)...
The Leyden jar is an early device for storing electric charge invented in 1745 by Pieter van Musschenbroek (1700-1748). It was the first capacitor. Leyden jars were used to conduct many early experiments in electricity.Homemade capacitor (Leyden jar) - Link.A typical design consists of a top electrode electrically connected by some means (usually a chain) to a metal foil coating part of the inner surface of a glass jar. A conducting foil is wrapped around the outside of the jar, matching the internal coated area. The jar is charged by an electrostatic generator connected to the inner electrode while the outer plate is grounded. The inner and outer surfaces of the jar store equal but opposite charges.
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From past experience, a two liter pop bottle Leyden jar is enough to give a decent jolt.
One time in eighth grade science class, there was some opportunity for extra credit, and one of the things you could choose was to make a leyden jar from a pop (some of you call it soda) bottle. Mine, based on the suggested design, had a foil inner electrode (made by cutting off the narrow neck and then replacing it later).
So, the time comes to test it out in class, by getting on an insulated platform and holding it up to the Van de Graff generator (now there's a fun toy!), and our clueless teacher has me hold it not by either electrode, but by the insulating part, and charge it from the VdG generator. I'm already thinking, WTF, you have to ground the other side of the capacitor to actually charge it.
Well, anyway, somehow the clueless teacher determined that mine didn't work and I went back to my seat with my Leyden jar. At which point, I was holding the jar by the outer electrode, and I absentmindedly touched the inner electrode with my other hand, and before I even made contact with it, I got a pretty memorable shock. Seemed pretty effective to me.
FWIW, A water inner electrode is a lot easier to construct.
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My physics teacher shocked me with one of these in my 5th year physics class (Thank you Mr Fenton!).
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I've also built some rolled capacitors using aluminum foil. I was interested in making high voltage capacitors that are quite expensive to buy.
In just a few minutes I got a 19nF / 10KV capacitor, see some tests/construction details here:
http://www.pocketmagic.net/?p=798
The only drawback is that homemade capacitors are a bit unstable.
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