Split My Bill app for iPhone and iPad
Split My Bill allows you to divide a bill fairly among a group of people. After inputting each item (and who it was shared with), the tax, and tip you want to leave, it shows you what each person will owe.
You can then email or text everyone the final amounts and easily track what you owe or are owed.
This application also has support for coupons, entering tax and tip as a percentage of the bill or in flat dollar amounts, and allows you to text or email out the final amounts to everyone.