Trover – lets you know what cool sightings, must see, must try activities are near you. I use it all the time and I have made contributions that allowed others to enjoy my findings all over the world!
Urbanspoon – great app to let you know what is near for places to eat and how much it costs with reviews.
There are also others for languages, hotels, even Instagram has helped but I use Trover the most. What are your favorite travel apps?