I built a trebuchet as a favor to the local boy scout camp, since they are very nice to me, letting me use welding equipment and such. They wanted something that the kids could use during "medieval days". My trebuchet was about 4 feet tall, made of oak, with a 7-8 foot arm, used 70 lbs of counterweight, and threw a baseball 500 or so feet. Pretty decent, considering its size.
Unfortunately some guy at the scout camp forgot to bring a weight set, so they only had about 20 lbs on as counterweight when they actually used it.
I also built one back in 6th grade for a science fair project. That was 3 feet tall and threw golf balls 400 feet, with 40 lbs of counterweight.
I like to make things that throw stuff. Spud guns, trebuchets, etc. Next, I think I want to build a gauss rifle.