I have the worst time knowing how to tip and how much to give. Here’s a handy little guide from over at Daily Infographic with Tipping Etiquette Around the World. While it doesn’t cover nearly all of the world (and really, what infographic could?) it does well for those that the dozen or so that it does cover.
It also breaks down information on tipping in the US with info on how much to give to waiters, coatroom attendants, hotel doormen and more.