There are many who think 9/11 was a conspiracy by the government to take away our amendment rights and go to war (Afghanistan, Iraq).  Since I take any theory seriously, I will post up the various reasons that have been proposed all across the web on why 9/11 was an inside job, as well as information that debunks some of the information seen below, you can make your own decisions whether you believe or not.

9/11 witnesses
firemen, police officers and media reporters heard and saw multiple explosions going off inside the towers, long before they actually fell which indicates there was possibly a controlled demolition.  

Demolition "squibs"
Squibs (focused points where a charge is set and debris is bursted out) shot out of the window before the buildings collapsed.  This can be seen in many tapes of the 9/11 collapse.

Buildings fell near freefall speed
All three buildings collapsed at near freefall speed. Building 7, which wasn't hit by a plane and contained only small scattered fires, came down symmetrically in 6-seconds. 6-weeks after 9/11, liquified steel was found under the rubble of all three buildings and the temperature was still in excess of 1500F. Jet-fuel simply cannot burn hot enough to reach that sort of temperature. Many of the steel beams found in the rubble of the three buildings were cut diagonally in exactly the same way as shaped-charges slice through steel diagonally to bring a building straight down.

Residue
Sulfur residue was found on the steel beams. Sulfur used with thermite is called thermate -- which produces even faster results than thermite, reaching temperatures of over 4,000F. Red-hot molten steel were seen dripping from windows prior to the collapse.  

Built to withstand Plane crashes
Les Robertson (WTC structural engineer) and Frank A. DeMartini (Manager, WTC Construction & Project Management) built the WTC with plane crashes in mind, which indicates buildings collapsed with a controlled demolition.

George Bush's Reaction
When the towers were hit, Bush was at a school visiting schoolchildren, and when he was told of the event, Bush didn't react at all anything and continued to read with the children.  Bush should have reacted by leaving politely and going to address the public about the incident.  Theories are that he knew before hand the towers would be hit, and he also knew who did it. His excuse for not reacting is he didn't want to upset the children.

Secret Service?
The secret service did not rush in to secure the area or move him to a secure location with the "surprise" attack, they did nothing.  Theories are that Bush and his men knew beforehand he was not a target by the terrorists.

Bush saw it before it happened?
President George Bush claims to have seen the plane crash into the South Tower at the time in which in happened even though the first crash wasn't televised at the time.

Rudy knew they would collapse
Rudy Giuliani reveals that he had been given advanced warning that "the World Trade Center was going to collapse"

Popular Science Article that debunks some of the theories concerning 9/11.  Click here