Basic Catapult Design  This Scouting site contains good instructions for building a basic, man-powered catapult. Nice if you want to launch water balloons...

Catapults: Weapons of Destruction  Good student site with some pretty detailed info and a couple of good pictures.

Golf Ball Catapult  Very nice site with some basic instructions on building a mini catapult. The detail work on this mini is incredible, and unlikely to be reproduced by a novice.

iCatapults  Lot of good plans for small/medium catapults. Light on the illustrations, but enough detail to figure it out. this site is hosted on a free server, so expect some quirks when accessing it...

The Realm of Taergerasmob  Good site with a lot of siege engine designs, from the basic to the forget-about-it. Lot of trebuchet designs here, too!

Knight's Armoury  Blueprint city! Onagers, Ballista, Trebuchets, Catapults other beautiful tools of mayhem.

Catapult Defense  Well, anyone who builds something large and destructive is going to have to come to terms with the fact that someone else is gonna figure out a way to defend against it. This page is dedicated to the fine art of not letting siege engines spoil your day...