Traditional calculators are fine, but you can easily lose your place, forget what you're calculating, or just waste brainpower on keeping track of everything. These simple apps for Windows and OS X make complex calculations a lot easier by mixing a notepad and a calculator, letting you tabulate everything in plain Enlgish.
When I'm performing a more complicated calculation, I usually open up Notepad next to the calculator so I can keep track of which numbers correspond to each real-world item they represent. If I'm adding up money totals, I'll write down the tax, shipping or whatever else I need. These apps -- OpalCalc for Windows and Soulver for Mac, iPhone and iPad -- turn calculators from cryptic number machines into actual, easy to use "logs" that automatically tally up any sums, products or other calculations you make as you go.
OpalCalc and Soulver arrange themselves as two-pane windows. On the left side, you type in the information you want to calculate -- say, how much it will cost you to build that new computer. You can type in dollar values along with descriptions of each component, so you remember which number corresponds to which item. Then, at the end you can add the shipping cost and a percentage value for tax. Without doing anything, you'll get the total dollar value of your purchase in the right pane.
It's a simple example, but both programs can perform much more complicated options when you need them to. You can assign variables (length=10inch) and then use them later ($10 x length), calculate percentages ("32 as a % of 80"), and lots more. The language is a bit different in each app, so check out OpalCalc's documentation and Soulver's feature list for more info. Check out the video above for a demo of OpalCalc to see how it works.
The Windows app, OpalCalc, is free for limited use -- five lines per calculation -- or you can donate any amount of money for the full version. Soulver for Mac is a bit pricey at $US25 (with a 10-day free trial), though you can grab the iPhone version for a paltry $4.49 and the iPad version for $6.49.