I asked myself that again last week while in New York. Taxis in Manhattan have a handy screen where you can choose the amount you wish to add as tip: 20 %, 25 %, or 30 %. If you want more options, I guess you better have cash on hand.
The tour guide through the UN building was offered a tip from an eager participant. She declined but vehemently hugged her.
Here in Texas if you don't like the service you simply give double the tax, about 16 %. If you like it, 20 % upwards it goes.
In Germany, we usually rounded up to the nearest two or so Euros. Base salaries for the service industry may be a bit higher though.
In Spain, my sister-in-law told us tipping was completely out since the crisis. Nobody had money to tip with, and waiters didn't expect it anymore.
In Mexico, everybody expected a tip whether we asked for the service or not. People would simply grab my grocery bags outside the store and insist on putting them in the trunk for me, or wave me out of the parking spot blowing on rather annoying whistles.
If you're an expat, the tips you are used to in your home country may be far too much, way too little, or even insulting in your host culture. Service levels will most certainly show discrepancies. The locals may also make exceptions for you and expect extra high contributions, assuming that as an expat your living standards are considerably better than their own or that of the local clientele (which may or may not be true). CNN wrote an article about international tipping a few years ago, and USA Today published this handy tipping chart you might want to consider as a basic guideline.