My college roommate from back in the 70s had a poster on his wall that read, "Just because you're paranoid doesn't mean they are not out to get you."
With that in mind, I think just because so many wingnuts espouse lunatic fringe conspiracy theories, that doesn't mean some such theories might not have some basis in fact. The question is, which such theories do you buy into, at least to some level?
To get the ball rolling, here are three I accept to some degree or another, although admittedly not for the reasons our nutjob friends might:
1) Government "Cover-Up" of UFOs--Of course there are things the governments of the world know about UFOs that are highly classified. I worked black a/c programs where our planes were reported as UFOs. Naturally, the USAF didn't go on national TV and say, "That UFO reported last week near Bodfish, CA, was a classified a/c, code named "Tacit Blue." Even if we accept there are extraterrestrial UFO, I believe the governments of the world keep what they know classified. We certainly keep classified what were know about potential enemies on Earth, so why would we approach non-Earthly potential enemies any differently?
2) TWA 800--No question in my mind this a/c was brought down by a missile, most likely a shoulder-launched MANPAD (Stinger, SA-16, Blowpipe etc) fired by terrorists. Commercial airliners have been brought down, or at least fired on, by terrorists with missiles numerous times over the past thirty-plus years. Just a few years after TWA 800 was destroyed, an attempt was made to take down an Israeli airliner with MANPADs in Kenya. The theories about a US Navy warship, or even goofier, a USN sub, taking down TWA 800 are bunk.
3) The US Knew About the Pearl Harbor Attack Beforehand--Not so much a conspiracy theory talked about on C2C, but a longstanding subject that, when viewed sixty years on, makes a lot of sense. Bottom line for me, the US knew the Brits were hanging on by a thread, and needed a way to get into the war. Hitler wasn't going to be stupid enough to add the US to his list of enemies, he was already fighting the Soviets and the Commonwealth. Conversely, Roosevelt knew the American people wouldn't unite in support of the war just to save the Brits. The attack by Japan provided the impetus needed to get us into the war.
Note, I'm not suggesting this topic necessarily to debate specific theories, although I'm sure that's going to happen. What I personally am more interested in is to what degree you buy into any conspiracy theories and why.