EMP (electro-magnetic-pulse) bombs can be made for $400. I was hesitant to talk about this (as per my earlier prohibition against writing a book on the theory of this type of warfare) but Popular Mechanics published it (they were stupid and shouldn't have). These have been around in military special ops for a long time.
The concept is simple. You can make a bomb for $400 using simple technologies that knocks out electronic devices for a half a mile (est) around the detonation. It turns motherboards into crispy critters, wireless phones/cells into scrap, and blows TVs. If the terrorists had used this type of bomb in 12 or more locations on 9-11 (three in NYC and one in every major city including Silicon Valley) the frenzy and the confusion after the attack would have been huge. People would not have been able communicate.
An added effect is that a sufficient number of these devices in an area can blow electric and phone switches. EMP can also knock fly-by-wire planes out of the sky (F-16 and other aircraft) and incapacitate cars. If combined with remotely piloted vehicles or ultralights, they can be deployed quickly from a distance and maximize impact range by doing an airburst. [John Robb's Radio Weblog]