Updated Feb, 5th 2013
Perhaps it's just my friends but we seem to have problems dividing up the check. One person had dessert, one didn't drink and other showed up late, etc., etc., etc.
With this app, you could put in the amount, add the tip (drop down based on percentage) then divide amongst everyone (default being equally divided) with a slider to adjust for paying more or less. Slide more on one would adjust less for the others.