[mct]

When I'm travelling internationally, a currency converter is always a good one to have.

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.

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.

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.

Good luck!