You can find the most important documents here, primarily the reports from the 9/11 Commission and the National Institute for Standards and Technology.
It is an impossible task to choose just a few of the many thousands of web sites, all pointing to alternative explanations of the events. New web sites appear all the time, while old ones disappear, often after a short while. Usually these sites are run by individuals, who give their own personal views.
As a rule, it is not possible to refer directly to sources on the web site for the Danish members of the Truth Movement, i11time.dk, since the movement by default does not allow the public insight into their debates or documentation.