The HP 12C is my all-time favorite calculator. I love its simplicity and size, I love the touch of its keys, I love reverse Polish notation (RPN) ... Did I lose you there? RPN is a more natural way of entering equations on a calculator (see examples at calculator.org). The HP 12C can also calculate stuff like interest rates and depreciation—perfect for a finance dummy like me. —Joi Ito, founder of tech venture-capital firm Neoteny.