My grandpa was a Freemason. Don't ask me questions about what went on and how high up he was because sadly he died when I was fairly young. By this Illuminati train of thought you guys think he was actually evil and taking over the world? If yes then I'll be sure to bring that fact up at thanksgiving.

Also another one of these organizations "taking over the world" I happen to be a member of. I'm not to sure if you guys also believe this but many people online who talk about the Illuminati also think Boy Scouts are contracted with the military preparing for marshal law and yada yada yada. Like CChaos said these beliefs only form because people are naturally suspicious of what they don't know/understand. I'm the highest rank a scout can be so if we were up to something I would know. Heck even Scouting has its own secret society within scouting called the Order of the Arrow but I can't expand upon that.

I think this explanation is a bit off. Especially in case of the illuminati, the (still strong) fascination stems from the fact that in their case, it is proven that they indeed tried to secretly take over power by undercutting government, trying to place their people in key positions etc., deceiving new members about their true aims etc., and being at least partly successful at it, in just a couple of years. And it was exactly after this became public that a widespread hysteria towards secret societies came upon the population.
In fact, if you look at what's nowadays considered verified knowledge about the illuminati, it seems that it was their proven aims and methods what later became the blueprint of every conspiracy theory about secret societies.

And it's no wonder that there are so many conspiracy theories about intelligence services, secret societies etc. Because they are indeed all conspiracies by their very nature, according to the true meaning of the word "conspiracy" - which goes far beyond what the common understanding of that word nowadays is.

Well, there's a lot of truth to this, but it's only one side of the truth.
The other side is that secrets and conspiracies have always been considered necessary (think intelligence services for example), and over the course of history, useful strategies have evolved for keeping secrets secret and managing complex operations that require lots of potential traitors. There's never a 100% guarantee of course, but the risks can be minimized.
Just to name a few strategies: Extensive tests for loyalty, pyramid-like hierarchies, the "need-to-know"-principle, deception, false information, threatening secret-keepers and their relatives, making them prone to blackmailing and and and. And even when secrets become public, there are counter-strategies like diverting from the truth, attacking the credibility of the traitor, trying to ridicule people who believe him, spreading several contradictory claims etc., which ultimatively leads to people being completely confused what they should believe.

That's not to say that the illuminati are indeed trying to take over the world etc. of course. I'm only trying to point out that these often-heard "conspiracies don't work"-arguments are not as evident as they may seem on first sight. Conspiracies are extremely common, and the vast majority of them never becomes public knowledge.