Your iPhone is only as useful for you as the apps you put on it. I'll do my best to highlight apps that I use myself and add value to my phone.
The first app I want to highlight is called Twist. How many times do we find ourselves meeting a friend, a significant other or a colleague and you want to let them know when you've left, that you've arrived, or that you'll be running a little late? Twist is an app that uses the GPS in your phone and based on your mode of transporation and traffic, will send text messages to whoever you're meeting and gives them updates on when you will arrive at your destination.
I use this with my girlfriend all the time. When you launch the app, you're prompted to start a new "Twist". It then asks for a destination. It can be a home address to which you may already have associated with a contact, or it could be an address or restaurant. You can type in the search field and it will auto populate results based on your location. Which is very helpful so you don't have to type in the complete address.
Once you select your destination, you can choose who to alert. Again you can search and the app will autocomplete based on who you have in your contact list. You can send Twists to multiple people as well.
And that's basically it! You can select how you're planning to arrive, either by car, public transit or by walking. There are also a couple options such as alerting the parties when you leave and when when you arrive but it's best to leave those options as is. You can also create favorite twists for common locations. For example, I have a favorite Twist set up for my home address which notifies my girlfriend since I always find myself letting her know when I'll be home.
Once you leave your location, the app will send a text to whoever you are meeting and will let them know your approximate arrival time. If you run into traffic, the app will even send an update text to let your party know you're running late and give an updated arrival time. Once you are close, it sends a final text to let them know that you're a minute away. No more texts of "I'm here", "Are you here", or "Where are you".
This is a hugely underrated app that I use as much as I can. Your friends and family will appreciate it since there's no more wondering where anyone is. It also keeps you from texting while you're driving (which is illegal in many states).
The app is free at the App Store.