PCalc [$9.99 - iTunes link] is one of the leading calculators for iPhone and now its computing power has been updated (as in universal binary) to include support for iPad.
PCalc is an (optional) RPN calculator that is great for scientists, engineers, programmers, and students. It includes a list of common constants, an ample amount of conversions, a paper tape that can be shared via email, and support for binary, hexadecimal, and octal calculations. You can also customize the look of PCalc with different themes and digit fonts.
The interface on iPad is rather beautiful in both landscape and portrait. The buttons are nice and big and everything is easy to read. Navigation between the menus is a much more pleasant experience than on the iPhone.
To see PCalc in action, watch the video after the break!