Rockstar Games has launched two free mobile apps that partner with Grand Theft Auto 5.
Grand Theft Auto: iFruit.
First up we have Grand Theft Auto: iFruit, currently available free for iOS only (but coming to the Android shortly). It basically gives you quick and easy access to Los Santos Customs from your mobile device. Using it you can customize your cars and also create custom license plates while on the run and then have your new ride ready and waiting for you next time you go to play the game.

The iFruit app also features a Tamagotchi like game in respect of Lamar's dog and Franklin's companion, Chop. Using the app you can take care of, play with and feed Chop, and as a reward the actual in-game version of Chop that's linked to your account will perform better. For example he'll be able to find items hidden within throughout Los Santos more easily.