For years I used Soulver, a Mac-based calculator app that let you write out calculations and formula, and it would render the calculations on the fly. It was a superb way to think out loud with numbers. One of things I loved is that it showed you the record of eƒach calculation, so you could visually follow your chain of mathematical reasoning — kind of like in the old days, with calculators that typed out each line of figures on a spool of paper.
Now Anton Medvedev has created Numbr, a free online calculator that works in a similar fashion.
In addition to letting you type in formulas, it does currency conversions, and — like Soulver before it — lets you carry the results of one calculation down to the next line. Again, this is great for letting you quickly march through some mathematical reasoning. You can also go back and tweak previous lines of calculation if you need to, and the changes will cascade down through your later figures, much as changing one value in a cell in a spreadsheet will alter the figures in any related cells.
Numbr and Soulver thus have a neatly chimerical vibe. They take some of the ready-at-hand ergonomics of a calculator, and merge it with the editability of a spreadsheet; so you get a calculator that offers some of what Steven Levy in 1984 famously called "a spreadsheet way of knowledge".
I'm gonna be using this a lot …