If you are travelling to the USA or Europe, consider the following tipping etiquettes. For other countries, ask the local hosts.
At a restaurant, always leave a tip. Tips can vary from 15% to 25%.
Waiter: 15% to 20% of the bill; 25% for extraordinary service
Wine steward: 15% of wine bill
Bartender: 10% to 15% of bar bill
Coat check: $1.00 per coat
Car attendant: $2.00 to $5.00
Remember that the amount you tip reflects the total price before any coupons, gift certificates, etc. Just because you get a discount, does not mean that your server did not serve up the full order.
If the owner of the restaurant serves you himself, you should still tip him. He will divide the tip among those who work in the kitchen and dining room.
Source: United States Etiquette Guide