Here's why the "chemtrail" conspiracy theory is bullshit:
The very existence of the Grand Conspiracy required to administer chemtrails may be challenged on the basis of the difficulty of maintaining such a conspiracy for so long. Aerial spraying from high-altitude would be an extremely inefficient and inexact mode of delivering chemicals to ground-based targets.
Pesticides, for example, are often sprayed on crops from airplanes flying at 30 feet. Wide-area mosquito spraying may be done from an altitude of 150 feet. Chemtrails are supposedly deposited at 30,000 feet or higher, where winds would likely disperse them unpredictably and the compounds would probably break down before reaching the ground.
If there was a clandestine campaign to randomly expose every single person on earth to foreign chemicals, it would be simpler and more effective to put them in the water supply (flouridation and stuff, you know), or the food supply — or if inhalation were necessary, to release them from ground-based vehicles.
Note also that if governments used just military planes and not airliners it would be a waste of resources for so little — if anything — in return, as they'd have to use a lot of them. Using airliners has the problems of why none of the many passengers, pilots, and especially technicians who maintain and fuel them notices something weird going on, and how both the chemicals and the system used to spray them are extra weight, something of concern for airlines because of their obsession with fuel prices as well as for aircraft manufacturers as they'd have had to incorporate that extra weight on their designs. And yet even another flaw with this conspiracy is why there are no screw-ups of the style of the spraying systems turning on by accident when a plane is taking off (Or, on a sadder note, when an airliner has an accident and its remains are scattered over a wide area why there're no traces of the deposits with the... stuff used for chemtrailing, or the latter itself).
In addition, said Grand Conspiracy would require that all of the world's governments, including the supposedly hostile nations that [insert your favorite government overlords] would have to be in cahoots with, would be agreeing to spray the chemtrails over their own populace, since there are very few national airspaces that do not have at least some aircraft flying to, from, within or over them.