Tip calculators are pretty common in most cell phones, but unfortunately the iPhone only has a regular ol' calculator (Which you can also use to calculate tip, but that's beside the point). Danny Goodman has developed this neat little tip calculator for the iPhone--enter in the number of diners, the total, tax, and it'll calculate the tip for you, plus the total amount of money each diner needs to pay up. I really liked that with each tap of an empty field, a large number keypad pops up, making it easy to enter in the numbers.
iPhone link:
http://dannyg.com/iphone/tipCalc/
