I am the sole developer of "OpalCalc" for Windows XP/Vista/7 (requires .NET 3.5 or higher), a program with a short but colourful history. It's been featured on LifeHacker, PC World and MakeUseOf, and is currently rated 4.6/5 (20 reviews) at Softpedia.
But since then, it's got quite a lot better
Here is the download page:http://www.skytopia.com/software/opalcalc/
The standard Windows calculator falls short in a number of ways, but most of all it won't allow multiple calculations at once, or allow you to go back and edit previous sums. Before OpalCalc, it turned out nothing on Windows like this existed. The Mac had something called 'Soulver', but a port didn't seem likely soon, so I've slaved away solid for almost a year creating something I hope many of you will appreciate.
The video on the site gives a good overview, but briefly:
Opalcalc uses a special 'notepad' interface where answers are displayed next to the sums, and update live as you type. This makes it ideal for quick sums, or more involved calculations, perhaps such as shopping lists, a work timetable, tax return, math problem, programming debugging, and much more.
You can work with dates and times, money and currency conversion, units (+ conversion), percentages, and use custom variables and functions. OpalCalc supports dozens of in built functions, full colour coding, printing/loading/saving, advanced notation support, excellent documentation and much more.
The pricing is interesting and I think sympathizes with the philosophy of donationcoder.com. Users can use the generous 'demo' (5 lines, but no time limit), and if they upgrade, they can pay a fee of their choice. Even 1 dollar would get them the full version. Furthermore, there are no passwords, keys or DRM of any sort - the license lasts forever, and upgrades are free.
Any comments and reviews would be appreciated. Feel free to dish out any constructive criticism (the amount I've had so far has only allowed me improve OpalCalc, so it's all good!).