but first things first... His supposed denial of responsibility...
Some have said that Osama denied responsibility in the beginning, but few say that the source of that is Karachi Daily Ummat in Pakistan, a country which has not always seemed to be very pro-USA or pro-West. That was proven when Osama found and killed there if not before. Karachi Daily Ummat is "believed to have close connections to Islamic groups in Afghanistan" according to an archived link from Ananova you can read here. Also, the newspaper says it "submitted questions for bin Laden to Taliban officials and received written replies". So, how much faith would you put in that? Me personally, I don't put any faith in this at all.
Then in December, 2001 already, Pentagon released a video showing Osama bin Laden talk about the 9/11-attacks and what his expectations were, that Atta was in charge etc. as you can see here. I linked the whole 59-minute video there. Over half (the latter part of the video) is of outdoor scenes seemingly unrelated. But in this full video, you can see the first parts when Osama and others enter the room and sit down before they begin talking about 9/11. It has subtitles.
So, this obviously contradicts his supposed "denial", where he also supposedly said it was against Islam to kill innocents, which is true, but he had advocated that it was "OK" to kill civlians (even going so far as calling it a "duty") in statements prior to 9/11, such as an edict from February 22, 1998, as you can read (among others) in this link from PBS's "Frontline". If you search for "civilians" there, it's the 4th hit of 5.
Finally, in 2004, Al Jazeera showed a video sent to them and published the transcript of said video, which you can see by clicking "Al Jazeera" above.
Fox News reported on this saying he for the first time claimed responsibility for the attacks, and also said why. This video was done prior to the election in 2004, and was likely an attempt to affect the outcome. Fox's link can be seen as sort of a "summary" of the speech and maybe "easier to read", which is why I link that too.
Likewise, CNN reported on this video, which was very special in that Osama admitted guilt concerning 9/11 for the first time. CNN's article can also be seen as sort of a "summary" of his speech for those that don't want to read the whole transcript.
This clearly shows that Osama at least claimed responsibility for 9/11. That, in itself is not proof positive of having commited 9/11, but a strong circumstantial piece of evidence I would say.
Obviously, one has to be aware that the video was to some large extent propaganda and Osama obviously had several purposes with releasing the video. All he said should be taken with a grain of salt, but it's still interesting to hear it from himself.
It's also good to have this information to show truthers so they see that Osama indeed admitted to 9/11, while his supposed "denial" is very much in question.