If you're planning to travel and dine internationally, here's a must-visit website to bookmark.
TipThisMuch.in has a list of tipping customs in a range of countries, from the U.S. (15 to 30 percent of the bill) to the UK (10 to 15 percent of the bill) to China (don't tip) and beyond. (There's even joke option for the planet of Gallifrey, mentioned regularly on the BBC program Doctor Who.)
Even better, the site has a calculator for figuring out the tip and then dividing the total bill by however many people are in a party--particularly useful after several bottles of wine.