Among some of my friends, I'm the right wing conservative, because I don't think Bush and other top US officials purposefully caused the collapse of the Twin Towers. So far, the conspiracy arguments haven't persuaded me. A good rundown (from the wikipedia neutralish point of view) of the conspiracy theories is here.
The conspiracy theories often consider the 911 Commission to be part of a coverup. I've thought the 911 Commission did a good job of explaining what happened and making recommendations, but I've disliked its unwillingness to assign blame. My earlier thought was that Clinton clearly did a better job at fighting terrorism than Bush, so the Dems on the Commission could have gone after Bush, but they didn't just so they could achieve unanimity on recommendations.
My nod to conspiracy theories takes things one step further. Even if Clinton did more than Bush, he wasn't perfect. The 911 Commission could have been protecting Democrats as well as Republicans when it refused to assign blame.
As for other evidence in support of a conspiracy, there are some open issues still, but none of the claims have convinced me.