There is a simple paradox I've noticed in product design; the objects with the most direct functions have a greater amount of accepted options in the marketplace. Two excellent examples of this are the chair and the watch. It would be difficult to think of two simpler functions and you would be hard pressed to find products with more choice in form, execution, and price.
In the 1970s, the simple calculator was one such item, as ubiquitous as they were varied. Vintage Technology has 128 examples on display with detailed specs and reviews. Worth it just for fantastic names like The Adman 830 and the Intercord Electronic 101! Some of my faves are above, but be sure to check out the full selection here.
Thanks to Jeremy Savage on the discussion boards for the tip.