Here's a piece from skeptic.com (you'll have to scroll down a little; it's called The Truth Is Out There...Way Out There by George Case) detailing why conspiracy theories are bogus by definition.

Case points out that conspiracy theories depend on hidden connections between apparently unrelated events, claims that sources of information like the media or the government are manipulated, and that some dark force in the shadow really controls everything. Here's his conclusion:

Conspiracy theories do not admit the glum, unresolved reality that public and private officials of good will may make single mistakes that spin vast webs of unintended consequences, nor do they allow that the likelihood of a few cynical individuals covertly trumping the infinite variables of human and organizational interaction, and never getting caught at it, is pretty slim. For all their curiously gratifying implications (“We didn’t lose; they cheated,” sums up Daniel Pipes), they permit us to forfeit our rights as engaged, aware citizens by insisting on a permanently skewed, nothing-is-as-it-seems order. If conspirators are running the world, then why bother to read, vote, think, discuss, act, progress? Today, more than ever, we should be demanding straight answers to our questions. Whether or not we think of them as “conspiracy theories,” glib brushoffs about Hidden Connections, about a Manipulated Media, and about how They Know Everything, are no longer good enough.

Note that certain elements of the Spanish Right have fallen prey to conspiracy-theory thinking. For example, some writers have been pointing out small coincidences and minor discrepancies in the March 11 bombings case, and using them to question the general truth of the discoveries of the official investigation. These writers have discounted the honesty of the police and the courts in the March 11 case, claiming that they have been manipulated. And, of course, the Great Satan behind everything is the PSOE and its minion Jesus Polanco. I have heard it said several times already that the Forum Filatelico-Afinsa crash is somehow the work of the Zapatero administration. Please. That is crazy.

Come on, people, the PSOE is obviously our political enemy, and we want to defeat them in the next election. They've done such a crappy job that we ought to be able to beat them without conspiracy theories.

I'll point out that the position of the PSOE and the Spanish Left in general on foreign affairs is just as steeped in conspiracy theory. In their case, of course, the Great Satan controlling everything is the United States and its minions Blair and Aznar; the media is controlled and manipulated by the Great Satan, as Noam Chomsky so bravely proclaims; and lots of small coincidences and minor discrepancies prove that the Bush family, in league with Halliburton, the CIA, the Jews, and the Bavarian Illuminati, actually hired Mohammed Atta to fly those planes into the World Trade Center so they could seize control over whatever portion of the globe they still don't control.