7. Tipulator
Don't overpay or underpay
Leave 10% on the table after a round of Sangria's in Spain and your generosity will cause offence. But if you do the same in New York -- not only will you be called a cheapo, but you'll be 86'd for life. Thankfully, wherever you go, there's the Tipulator app for iOS, a nifty tip calculator to help the mathematically challenged split bills and tips while saving yourself social embarrassment.
There's an Android Tipulator too, and while it's not as sleek as the one developed by the iOS developer Sophia Teutschler, it still gets the job done.