You can't throw a rock in the App Store without hitting a Tip Calculator. There are plenty of both free and paid ones to go around. My current favorite is CheckPlease (iPhone | iPad). These Tip Calculators work great for me here in the US, the problem I have is when I travel abroad not knowing what's customary and what's expected in other countries. In some countries the tip percentages are much lower than the 15%-20% here. Other countries you're not expected to tip at all (and they look at you funny when you do).
Tipping Guide is the answer
Tipping Guide at first glance appears to be a global directory of tipping practices and while that would have been worth the price of admission alone for me, I later discovered that it is in fact a tipping calculator too. While it won't replace CheckPlease for me here domestically (because I like the CheckPlease interface ten times more), it will definitely be both my guide and tip calculator for my international travels. Also no data connection is required. The Tipping Guide info is built-in.
The Bottom Line
This App is just what I was looking for! It has the best UI (user interface) of the ones I've seen to date. The only real reason I can't give it 5 stars is because while the built-in Tip Calculator is functional the UI for it (the calculator) is kinda lame. You have to use the spinny wheels to enter the amounts. A standard numeric keypad would be much better and FASTER. With that said, having a tip calculator built-in beats switching between Apps any day. Aside from the Tip Calculator UI, I love this App!