Tip calculators exist...usually as a function on a simple handheld calculator.  Haven't seen one in a while though. 

Your best bet is to use simple math in  your head though:  If the bill is for $80.00, just move the decimal point one number to the left...which means that you have $8.00 for a 10% tip.  Multiply by 2 for a 20% tip or divide that number by half and add it to itself for a 15% tip. 

You could also just double the tax...so if tax is 6%, multiplying by 2 gives you a 12% tip.  This works better in NYC where tax is 8.25% 

I have a point system in restaurants, which often results in an "off number" like 7% - so what I do is move the decimal place over by two places...so .80 is 1%...so then I just multiply by 7, which gives you $5.60 (7% of $80).  Easy.