Last Updated Jul 5, 2007 6:15 PM EDT
It's hard enough determining how much to tip cab drivers, waitresses, porters, etc. in the States; it's even more confusing when you travel.
Get Rich Slowly has a handy guide for folks who are staying within the US. (And in the comments, Metafilter's Matt Haughey issues the friendly reminder that big tips often equal better service at places you frequent.)
Overseas, things can be different. Americans tend to overtip by a lot, which can lead to embarrassing situations in certain countries (most notably: Japan). Keep in mind that many hotels and restaurants in foreign countries already add in service charges, and that directly giving money to a service provider (i.e. a waiter) in some countries can be considered rude. Here's a general rundown from Magellans about how much to tip for meals and taxi rides in lots of different countries.