I decided to make a trebuchet for the Brick Builders contest and here it is. This one is all Lego, no tape or string or anything but Lego pieces. If you like the project I would appreciate your vote.
Also, please excuse the mess- I'm in the process of moving and the whole place is a disaster.
Step 2: Throwing Arm
Step 3: Weight
If you can't find these perhaps some old lego motors or the old RCX 1.0 would work as weights if you have them. Or you could always cheat and use non-lego parts.