Apple's built-in iPhone calculator will suffice for roommates trying to divvy up monthly bills, but for heavy lifting, you'll need Calcbot. The app makes quick work of trig functions, roots, and factorials. There's also a helpful live expression history for keeping track of unwieldy operations.
WIRED Shows a complete history of your calculations. Makes a satisfying click noise for key presses.
TIRED No memory button for storing answers.