The Google Places iOS library by Lukas Kukacka uses a simple block interface to interact with the features of the API that include mapping, localization, and reference. This project offers place search, with nearby search and text search. It also provides places details, with a more detailed information about a specific location. Available as CocoaPods.
Back in February 2010 we asked Why can't you share photos on Foursquare? We discussed that this could be seen as an oversight or an opportunity for others. However, this question has now been deprecated by Foursquare with the introduction of its add photo API method, which suggests that the previous absence of a method was simply because it wasn't a priority, it was an oversight or the opportunity hasn't been taken up by developers as Foursquare had hoped. In addition to the new add photo method, Foursquare have also introduced API methods which allow comments to be added and deleted from checkins. These methods have already been rolled into the official iPhone and Android apps along with a few 3rd party apps that use the Foursquare API.
Check-in app Gowalla puts a fun twist on location sharing by helping its users discover wonderfully-designed virtual items that can be found, swapped, and dropped all over the place. Developer Ben Dodson used the Gowalla API to put together Wallabee Travel Edition.