I've always liked how minecraft cannons were built, but they were always too wasteful and time consuming to be used realistically. I set out to change that. Cannon powder works exactly like tnt, except it doesn't remove blocks or damage entities. It's used to push things. I also hooked up the dispenser to spit out live TNT / CannonPowder instead of just the item.
You can use it to create cannons, elevators, traps, etc.
The recipe is shown in the video.
Visit my minecraft forum thread for discussion and feedback.