Carl Banks has Gratuity (iTunes Link) for the iPhone and iPod Touch in the iTunes App Store for FREE (regularly 99-cents) for a limited time.

A clean & interactive tip calculator built for life’s most common tasks. Choose among six everyday services for customary gratuities. Adjust your tip amount based on the quality of your service. Easily split the bill with a parties of two or more. Not satisfied with a particular preset? Gratuity is fully customizable, where settings for each venue are left at your full discretion.