Being online for the last 15 years, and having a strong (if sceptical) fascination with conspiracy theories I've run into quite a few over the years.
Many are clearly and obviously ridiculous on the face of them, while others somehow suddenly turn from "ridiculous paranoid fantasy" into "boring history" in the public consciousness - usually (and oddly) without ever passing through the stage of "important and shocking revelation" in-between.
Obviously these days (after years of the X Files and similar cultural touchstones) "conspiracy theory" is a loaded and negative label, and most people instinctively disregard anything described as such. However, I think this is somewhat unfair - there are more conspiracies out there than people typically realise, and they've often played a much larger role in shaping the world than most people give them credit for, even starting wars, bringing down presidents and contributing to the maiming or deaths of hundreds of innocent citizens.
In addition to the "obviously idiotic" and the "obvious-with-hindsight", I believe there is a class of conspiracy theories which - while incomplete and mis-attributed - still conceal a nugget of truth and worthwhile insight, as long as you disregard their more fanciful claims.
As an example, with the rise in filtering systems and various countries' attempts to filter the net, the meme is gaining strength that these are simply cynical excuses by authoritarian governments to restrict their citizens' freedom, and censor the public discourse.
These concerns are persuasive in that they recognise the problems with such systems - that once in place they only tend to ratchet tighter, and that people will accept any amount of change as long as it's introduced in small enough increments. However, systems like censorship (and by extension even really huge conspiracy theories like the idea of the so-called New World Order - an internationalist/globalist conspiracy to dissolve national boundaries and unite the world under one global government) wouldn't necessarily even require a conscious conspiracy.
These trends (if they exist) aren't some Machiavellian super-conspiracy implemented by a smoky room full of the rich and powerful - they're simply the emergent behaviour of lots and lots of different people, all following their own, parochial agendas, who find themselves (often quite unconsciously, or inadvertently) all pushing society in a similar direction.
Returning to net censorship, what happens is that one short-sighted government puts a filtering system in place to filter out "unambiguously evil" content like child pornography, and then later on that mechanism is inherited by later governments, who have their own ideas about what's considered ban-worthy.
Successive governments only encroach on freedom a tiny bit from the previous government, but every time someone complains you get people shouting down dissenters on the grounds "it's only a trivial change, so why are you getting so bent out of shape about it?", or the ever-popular "Yeah, but
X is evil - how can you not want
X filtered out?" (where
X is "terrorism", "hate speech", "child pornography" or the current bête noire.
The other important part of this process is that it's a ratchet effect. Almost no government - short of massive upheaval like a revolution or regime-change - is going to ease off on the filtering, because firstly there's no political capital in doing so, and secondly it would make them look soft on terrorism/paedophilia/whatever the current reason is.
So you have a mechanism where controls ratchet ever-tighter, it's practically impossible to ever loosen them short of a major social upheaval, each step is such a tiny one that people can't emotionally appreciate the importance of resisting it, and anyone who does resist is easily dismissed as reacting disproportionately, or being actively in favour of terrorists, or paedophilia, or whatever the excuse du jour is for "just tightening restrictions a little bit, just this once".
Importantly, and this can't be said enough, this doesn't even require a Machiavellian conspiracy or a particularly authoritarian government behind it - it can happen simply by lots of honest but short-sighted people of good conscience just doing what they think is for the greatest good... but if allowed to run unchecked (and as previously indicated, it's hard to check it without looking like a lunatic or conspiracy theorist) it still ends up in a more restrictive, less free, more authoritarian state in the end.
Project this trend far enough ahead (a few decades is usually enough, although sometimes as little as one will do) and you can quite easily get from an open, successful democratic society to an authoritarian police-state with no large or jarring social upheavals required.
This is exactly why it's so vitally important to never, ever grant any additional powers to any government unless they're absolutely unarguably necessary, and even then grant them for a limited span of time, and never, ever renew them unless there's a proven requirement to do so (ie, never renew because it's the default position to keep the law on the books, as was arguably the case with the PATRIOT Act renewal in 2005/2006).
Plenty of people instinctively recognise themes and trends like these, but a common cognitive illusion called an overactive sense of external agency (PDF warning) causes them to mistake simple but counter-intuitive emergent behaviour for a conscious, intentional conspiracy. This makes them easy to dismiss as paranoid or crazy, and makes it easy for others to dismiss both them and any legitimate trend they've identified (an example of the Association Fallacy, also known as damning by association).
Clearly I'm not suggesting that all (or even most) conspiracy theories are realistic, accurate or plausible. However, if you run across one it's always worth making an effort to separate out the What and the How from the Who and the Why, and seeing if the processes and effects it describes have any validity on their own.
If someone tells you that a concerted cabal of international bankers and financiers are attempting to bring together and integrate the disparate economies of the world, dissolving national sovereignty and bring the world to heel under one world government made up of shape-changing lizards, you can safely laugh at the lizards.
However, shorn of its intentional (and sensationalist) nature, there is a distinct trend towards economic and political integration in international politics, the advent of the internet and international trade deals have inadvertently acted to make national boundaries progressively more porous, and increasing geopolitical integration necessarily reduces national sovereignty somewhat.
When you put it like that it's boring and mundane, but wild-eyed, crazy-haired conspiracy theorists have been pointing out the What of it since the 70s or 80s, and - vaccinated against listening by their kook-like presentation and the cultural stereotype of the "crazy conspiracy theorist" - most of us still aren't even consciously aware it's going on.
I find that extremely interesting, although I ascribe to it no particular group, agenda or intent.