No one knows on what fateful day man discovered that it was safer to kill his neighbor from afar, but that day saw the beginning of the great history of missile weapons. Over time, a rare convergence of technology and art created a machine whose simple elegance and ferocious power both awed and terrified: the trebuchet.
Since building my trebuchet and creating this page, my enthusiasm for the subject has waned. This page will probably be updated infrequently.
The trebuchet, or "treb," operates on a few very simple principles. A long beam rotates on an axle which is supported by a frame. To one end of the beam is attached a sling, and to the other a heavy weight of some kind, called the counterweight. One end of the sling is fixed to the beam, but the other is attached by some mechanism (usually a simple peg and loop) that allows it to be detached at some point in the firing. Before firing, the sling is on the ground and the counterweight is suspended in the air. When the trigger is pulled, the counterweight falls, the other end of the beam rises and the sling begins to whip out and up. At some point, one end of the sling is loosed and the projectile flies away, hopefully demolishing the target.
There are many types of engines that are designed for the same purpose as a trebuchet, namely throwing large rocks great distances, but the one with which the trebuchet is most often confused is the catapult. A catapult's workings are ugly and inefficient, while the operation of a trebuchet is beauty itself. All trebuchet owners know that a treb is superior to a catapult, and are ready to explain why at a moment's notice. This elitism is part of what makes being a trebuchet enthusiast so fun.
While building or demonstrating the operation of a trebuchet, one will invariably be approached by curious people who exclaim, "Oh, cool! A catapult!" These well meaning but ignorant onlookers can easily become an annoyance, and even the most righteous of treb enthusiasts may find himself imagining a new, human target for his engine. But remember, a trebuchet owner's first duty is to educate, and such a situation is a valuable opportunity to teach others, most of whom are eager to learn something new. As for the rest, well, they may still have value as counterweight.
My first attempt at a trebuchet is Tech Brute: a four-foot-tall
hinged-counterweight engine designed for throwing water balloons. As it
turns out, Tech Brute is capable of throwing more than just water
balloons. In fact, with even a modest counterweight, enough force is
generated to crush a water balloon before it can leave the sling.
Tech Brute's first act of destruction happened during some test firings
in the back yard. With about 80 pounds of counterweight, it had been
consistently throwing snowballs over the house and into the street
beyond. Trying to get more distance, I added another five-pound brick to
the counterweight basket and made a slight adjustment to the sling. The
next shot sent a tightly packed snowball crashing through an upstairs
bathroom window. It shattered two panes of glass and apparently
disintegrated, leaving only a bit of slush in the bathtub. Most of the
shards of glass hit the shower curtain, thankfully arresting what could
have been a very dangerous flight, although a few pieces found their way
out to the hall and down the stairs.
I made plans in a CAD program called QCad. Back then the program was only free software, but now they have a free "community edition" that they seem to be ashamed of. Anyway, as of 2009 you can still get source code for QCad 1, which I used to make the plans, from SourceForge CVS. It is probably possible to open the DXF files in other CAD programs.
The plans are in four pages. There is something wrong with page 1 (side view); when I found the files again it only contained a few lines. I guess it is lost forever. Dimensions are in inches.
All four pages: treb-plans.ps, treb-plans.pdf.
Bitmaps: 
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I later found some paper copies of the plans that differ slightly.
DXF files: treb-plans-1.dxf, treb-plans-2.dxf, treb-plans-3.dxf, treb-plans-4.dxf.
Materials list: treb-plans.txt.
All files in an archive: treb-plans.zip.
I first learned of trebuchets after reading an article in Smithsonian Magazine called Ready, Aim, Fire!, which was about two trebuchets constructed using medieval methods. I decided to build one after seeing Secrets of Lost Empires: Medieval Siege on NOVA, which was about the same two trebs.
The Grey Company Trebuchet Page is a comprehensive site with histories, explanations and plenty of pictures of the Grey Company's trebs.
Ron Toms's Trebuchet.com has several great stories and is probably the best site to visit if you want to stay hip to the state of the art.
siege-engine.com has an eclectic mix of information and pictures of many different types of siege engines. I like the trebuchet design for Dr. Seuss.
Many operators of trebuchets and other hurling devices practice their craft at the World Championship Punkin Chunkin. Some of the methods used to propel these gourds are truly amazing.