After seeing the EMP Hardening thread I figured I would bring up mad scientist levels of possible tech that could theoretically work, then again I doubt anyone would use it. I will post more on this later, as I am at work now. I will just put basic stuff that has exsisted before WWII or at the time of for now and eleborate on stuff that could also be wizard proof theoretically. The first item is probably the most popular, the Rail Gun. Feel free to add other crazy tech. That being said if there are some errors in my rail gun please correct me I want to be as accurate as possible.
Rail Gun: Born on April Fools day in 1919 (that was the day it was submited to the US patent office) the Rail gun is quickly becoming the favorite weapon of steam/diesel punk fans and would be a nightmare if an enterprising young wizard/ practicioner got hold of it and doom if a Fae queen got a hold of it..
How it works:
The essential design of a rail gun has not changed since the inception however powering the machine has always been a problem. The Solution? Magic. Rather force rings or force devices like Harry's ring and something to transfer that to the electrical power to charge the rail gun (although causing lightning to constantly hit something that absorbs energy would probably work just as well.) Daily walks and activity charges even on a mass scale isn't exactly the way to go here. Rather you need to charge them supersonically or something else to cause insane amounts of charge. This can be done by instead making these objects into nuts and bolts to moving parts on a car or into tips for a whip or tips of a helicopter both of which break the sound barrier. (And a helicopter quite frequently.) How you would do this would be probably up to the story teller to see if it's feasible (Or if the story teller is using it as a Macguffin.) These objects should be designed to fit together like a puzzle and function as a force battery to actully charge up the rails creating the magnetic field (Here is a nifty diagram from Wikipedia
http://en.wikipedia.org/wiki/File:Railgun-1.svg) Depending on the weight of the object (which will likely be smaller than a mail box but realitively dense) the projectile can go from mach 3 to theoretically around mach 10.
Problems: A rail gun shoots things at insane speeds which cause the gun itself to easily wear out. Then again there is the whole thing about the power of the gun causing plasma to fire around the projectile. Essentially that projectile is going fast, and you do not want to be near that gun when it goes off. Also when it goes off, everything is going to come down on that location from the local cops, to some 3 & 4 letter agencies (who would also probably like to see your designs) and probably every supernatural organization. This gets even more complicated with the size of the rail gun, which is quite big. So unless your dealing with something horrifyingly powerful it is not going to be worth making one. Almost forgot to mention the problems presented with an object traveling near other objects at speeds of Mach 3.5+, which are of course the matter of sonic booms and waves destroying all sorts of things. Collateral damage would be insane, and with the speed it's traveling its going to take a long time before it hits the ground even if it hits a target (Once it gets over Mach 7 I could safely say that it would probably go through anything including a tank and killing everything inside from the speed.)
Next up when I get on next break The Hwatcha (Unless of course someone wants to go ahead and do it.)