There's alot of people who believe such. There's actually some pretty intricate explainations, with some pretty strong evidence supporting such even.
I watched an hour long "zomg conspiracy theory!" video on it once on youtube, though I lost the link sadly. It was actually pretty in depth and quite surprising, since I doublechecked a few of the facts and found out quite a few were accurate and did lead to interesting suspicions.
For example, a large sum of the gold under the building went unaccounted for during the chaos of the attack, which may or may not imply that it was stolen during the attacks.
Another example being that the section of the pentagon which was damaged (which's commonly forgotten that it actually occured as well), had that particular section practically evacuated a few days before, with some marks on the lawn photographed several days before it happened in the exact angle the plane came in, and then there were some arguments about how the damage was consistent more with a missile than a plane, and compared various other accidents for evidence of such.
There were alot of examples, but truth is often stranger than fiction; there's alot of people who still believe the moon landings were faked, though much of their evidence simply only works with the assumption that the moon's gravity, atmosphere and soil are identical to the earths', which they aren't.
So, not sure if I believe that conspiracy theory, but it's not like the guy's all that discredited; honestly he's got more evidence to support his belief that the USA intentionally bombed their own stuff (and historical backing; the USA's tried to do that before actually, and there's several recorded cases of it being suggested, and nearly acted upon before being overruled in other situations), than bush had of there being nuclear weapons in iraq.
So really, he's not totally crazy, just probably using conspiracy theories to his own political advantage since he doesn't even have to do the work to make them up or get the evidence for such, as americans are doing all his work for him.
If anything, it's a pretty good move, politically, for where he's at. And it's not like anyone can fully disprove him at the moment, without blowing ridiculous amounts of resources into refuting each section of evidence one at a time, in which case he may just drop his case and go "oops guess I was wrong", and still win by forcing doubt for so long, and having taken several million to billion out of the economy to prove him wrong.
It doesn't matter if they're right. If they can't proove they're right, then they're wrong. No matter how right they may be.
The 9/11 commission had more gaping holes in it then it did mindless fluff, and the administration was caught red-handed in many a lie, most notably Iraq. The way the administration used it shows they exploited it, which further shows that they cannot be trusted on many levels...