NYC'ers are so savvy to their own streets there's little risk of cabbie gouging, but travel afield and you're suddenly vulnerable to your driver's pernicious whims, like running you in circles, or coyly suggesting, "bath house?" Refuse to be screwed, with WorldTaxiMeter.com.
WTM's a free, online and mobile service that aims to map taxi info across the planet -- vital for globetrotters wary of public transpo dangers like sarin gas, gypsies, and foreigners. Enter your starting point and destination (or choose from pre-set tourist landmarks) and WTM'll pop back route and fare, calculate surcharge rates and gratuity, even offer to convert the fare to Euros, Pounds, or Crowns (Czech yo' self?). Besides San Francisco, the current destinations are all NYC-accessible Euro burgs, including Barcelona, Madrid, London, Prague, and Rome, where you must do as they do, or risk being drawn-and-quartered by Vespas.
On the off-chance you're not familiar with NYC streets, WTM covers you here too, finally putting an end to all those cab rides where you'd ask for The Garden and "accidentally" end up at West Garden. Via Staten Island.