As far as I can tell, Soulver is the only real advance in calculator technology since calculators were invented. It’s a fundamentally different—and cleverer—way of doing math. Enter numbers and calculations as if you were writing them on a sheet of paper, and Soulver will take care of doing the math for you just like a calculator. With Soulver, though, you can easily combine the output of any arbitrary set of lines, updating the combined output when any of those lines changes, which makes back-of-the-envelope calculations a whole lot less traumatic. The iPad and Mac versions also ignore words following numbers, allowing you to easily label items in a budget or shopping list just as you would on paper.
Soulver Desktop has built-in conversions for standard items like distance, time, weight, cooking, and data units, and even connects to the web for up-to-the-minute currency conversion rates and stock and commodity values, so you can figure out the value of international trade deals—if that’s your thing.
I’m doing a poor job of explaining just how awesome and useful (and fundamentally clever) Soulver is, so if you do anything at all with math, I highly recommend trying it out. The Mac version has a free trial, and I can pretty much guarantee that once you’ve used it on the desktop, you’ll want it everywhere.
Also check out:
UPDATE: Those who find Soulver for iPhone and iPad too expensive may want to check out Tapbots’ just-released Calcbot; it’s not as capable or innovative as Soulver for iPad, but it keeps a history ‘tape’ of past calculations and should serve as a solid and inexpensive Calculator replacement for many people.