Tip Calculator does just that, from your iPhone. Or you could press the calculator button and add 15% to your total.

<a href="http://dannyg.com/iphone/tipCalc" title="TipCalculator for iPhone" target="_blank"><img src="http://www.modmyi.com/images/apps/tipcalculator.png" border="0" height="342" width="310" /></a>

<a href="http://dannyg.com/iphone/tipCalc" title="iPhone Tip Calculator" target="_blank">http://dannyg.com/iphone/tipCalc</a>