Good Spirits is a drink tracker for iOS. Ever since learning about the correlation between drinking and various kinds of cancer, I've been meticulous about logging and tracking every drink I consume. Good Spirits makes this process easy: just set a weekly limit, check in your drinks, and the app will let you know when you're in the danger zone. For the craft beer drinkers, you can automatically pull new check-ins from Untappd. Free and soon-to-be open source!
MusicMessages! is a collaborative step sequencer for the iMessage App Store (iPhone, iPad). Using the simple and (hopefully) intuitive interface, you can punch in a few chords or a percussion line and send the message off to your friends, who can then make their own changes and send it back. Five instrument layers and over 40 MIDI instruments are available to use. Bonus: if you have an iPhone 7, the note buttons respond to pressure and "pop" like bubble wrap with the help of the Taptic engine!
I made a custom keyboard for iOS called Translit Keyboard that lets you transliterate English into Russain, Ukranian, Armenian, Belarusian, Georgian, Greek, and Macedonian in real time. (Transliteration is phonetic: you type "raz, dva, tri, chetyre" and get "раз, два, три, четыре".) The keyboard is based on a simplified reproduction of the iOS system keyboard built in Swift, called Tasty Imitation Keyboard, that I have open-sourced. It could be used to build custom keyboards similar to Translit Keyboard fairly easily.