Curta mechanical calculator:
www.vcalc.net/cu.htm: The most complete site devoted to the Curta. A lot of information, pictures, diagrams and manuals.
www.curta.li: Beautiful pictures of the Curta, including special versions, with manuals and vintage advertisements.
www.curta.org: A lot of information about the Curta.
www.curta.de: In German.
Vintage Electronic Calculators: