What Are The Conspiracy Theories About 9/11?

Government foreknowledge: The US govt. knew the attacks were coming, but did nothing to stop them.  Either because they wanted an excuse to go to war in the middle East (settling old scores or putting money into the pockets of arms manufacturers), or for some other complicated economic reasons.

US government did it: The US government planned the attacks, to create a pretext for going to war against Al Quaeda, Iraq, other sin the middle East.  This is either because USA wants to dominate the region's oil, or just because the most influential advisors in Washington were still smarting about the Iranian hostage crisis in 1979 and wanted any pretext to get lots of troops in the region.

The evidence for these conspiracy theories is... How could the most powerful country on Earth not know about such a big plan of attack?  A  steel-frame building should be able to stay up after a large airplane crashes into it and starts a big fire (supposedly).  The videos of Osama bin Laden taking credit for the attack don't look perfectly like him. Actually, there are a lot of other bits of information offered up as "evidence", but most can be quickly disproven.