Tipping at a restaurant is typically 15% of the bill (before or after tax) +/- unless it is explicitly included on your bill already.

Just to further complicate matters...

It is pretty rare to have a service charge added to your restaurant bill - as they do in many parts of Europe. The one general exception is for a large party. Many restaurants will add a 15% tip to the bill for large parties - usually 8 people or more. So...if you're in a large group make sure you don't tip twice.

Restaurants: 20% tip was a starting point, but I marked it down for flaws in the service more easily than I would in the UK.

Maybe I'm cheap but my friends and I always disagree on this point. My rule of thumb is at least 15% for good service and 20% for very good service.

Another one I just thought of is rental car shuttles. If the driver handles your bags, $1 or $2 per bag is a proper tip.