The past couple of trips that I have taken have been to major metropolitan areas. New Orleans, Sand Diego, New York, etc. I have found that my iPhone has been invaluable in these areas when trying to find my way around the cities and finding things to do. Here is a list of my favorite travel apps (for the city)
I put this one first because I think it is the least well known of the bunch. Transit is a must if you are in a city where you will be using public transit. Open it up and you can instantly see all of the public transit options around you. What is really nice is that you can set your destination on the map and it will automatically calculate your public transit options for you. It also has real-time updates of routes for those cities that make that data available.
The split of Foursquare last year was a tough one, but to be honest I have found it to probably be good for the company in the way the apps are used. The new Foursquare is a way to find things around you and is more than just for restaurants, but that is where it excels. I like it over Yelp, because it categorizes restaurants and locations using a crowd sourced tagging system, which I think is very efficient. It is super easy to pop the app open and see what are the best tags at a place for food, etc.
Swarm is the other off shoot app that came from the Foursquare separation. I not longer find it quite as useful, but checking in is still a fun process, especially if you are separated from other people on your trip, it makes for a fun way to keep track of where someone has been throughout the day. I still miss my favorite app of all time, Gowalla so let's hope Swarm can take some of those old Gowalla ideas, like the geocaching aspect and make the game more fun.
Foodspotting is a great app that should get more attention. It has a bit of overlap with Foursquare in its purpose, but it's strengths lie in its reliance on photos. Instead of just reading about what is good at a restaurant, instead see what other people have taken pictures of and read how they liked it. It is a sort of app that if you are really into food, then it is a must have.
Not much I can say about this one, that I haven't already said. I store a lot of info in here and for vacation planning it is ideal for throwing all of your receipts, digital tickets, event lists, to dos and other stuff for your vacation in here. I even throw in .PDf files of maps or brochures which I can easily bring up on a moment's notice.