CalcPad is a math tool. It allows you to type calculations, and the answers are computed when you press Return. You can define variables and functions. You can also work with units (such as meters, seconds, etc.); all standard units are pre-programmed, and you can define your own. (For more info, look in the Help menu within CalcPad.)
Most users who have tried it so far say that CalcPad is far more pleasant to use than a standard calculator, because you can more clearly see what you're calculating and it's easy to go back and change values or fix mistakes. Many users also find that doing calculations with units greatly reduces errors that are otherwise easy to make.
I haven't officially announced CalcPad to the world; I'm looking for feedback on additional functions wanted, or (of course) any bugs found. Also, I've currently compiled CalcPad only for Mac OS, though I intend to supply Windows and Linux versions too.
CalcPad is free and open-source. Download it from here: <http://www.codenautics.com/calcpad/>