Want a good calculator with *lots* of features? What's that you said? You need to use scientific notation, or do HEX math for your programming needs? Look no further than the RealCalc Scientific Calculator. And it's FREE.
I love this calculator so much that I put a shortcut to it on my android desktop. First of all, it looks great. A lot of work went into the user interface on this application and it shows. If you have ever used a scientific calculator you will feel at home using RealCalc, since it has most of the same features, with buttons that have multiple modes, etc. It even includes conversions between units.
If you are an android developer, this is a calculator that comes in handy (although if you are using the same device to test your algorithms on as well as check the math, it may be a little awkward).
Here's a screenshot of how the button options change in HEX and then in BINARY mode:
If you are not a programmer, than you can still use this down at your local mall to quickly calculate the sales percentages of items on sale - just type in the original sale amount, then multiply, and type in the percentage sale (such as "50") and press the percent key. Wallah, your new price! Handy!