I operate the German Curta Page.
http://www.curta.deEditor's Note: Jan is too modest to state that his Curta Calculator information site is wonderful. The site is exclusively in the German language, but for English language readers he has provided plenty of pictures of Curta prototypes, exploded views of the Curta, and engineering line drawings that provide a superb view into the workings of the Curta. There is a page on the site that shows the logic functions of the Curta, allowing you to visualize what happens when you turn the crank.
Jan has also developed a Curta Simulator so you can operate a virtual Curta. The virtual Curta even includes the correct sounds! The link below is to Rick Furr's Curta Calculator site, where the simulator is also located. The text is in English.