When I'm travelling internationally, a currency converter is always a
good one to have.<p>You can try searching for public transit apps specific to the
cities you'll be visiting. Depending on where you'll be travelling,
there are some nice ones that have built-in maps of the transit system,
trip planning, and real-time departure estimates. For a city like San Francisco, you may need more than one to cover the various public transit providers available (BART vs MUNI, etc). Lacking a specific app, you may want to just bookmark mobile versions of the public transit provider's website.<p>I also like installing a maps program that can make use of offline data.
On Android, there's OsmAnd. That way, even if you no longer have data
service, you can use the GPS to get a sense of where you are and what's
nearby.<p>TripIt can be nice for keeping your travel itinerary easily accessible
-- but of course it's always good to have hardcopies either on your
person or in your carry-on.<p>Good luck!