There are many ways to keep yourself safe, and many other items you could purchase, but an app is one of the most convenient things to have as most people never leave their phone behind. Luckily, I attend a college where police escorts are the norm, but I still use my safety app at least once a day when walking home in the dark from the library.
1. Safetrek
This is the app I have been using since I've moved away. It's compatible with Android and IOS, and charges $3 a month, or $30 a year. It may sound pricey for an app, but it's given me a complete sense of safety, and that is priceless. Basically, when you're feeling unsafe, you open the app and hold down a button. The button tracks your exact location, and when you're feeling safe again, you release the button and type in your personal 4 digit code. If something happens and you don't type in your code, police are immediately notified to rush to your location. I like it because you can inconspicuously call for help without saying a word.
2. bSafe
This is probably the next best app, as it can connect you to friends. This app allows your trusted friends to track you, and see when you've arrived home safely. If you're in danger, you can press a button to alert your friends, start recording a video of the incident with voice, location, and time stamps. It also has an added bonus of a fake call, for dates going awry. All of this is free for IOS, but is not available for Androids. The only reason I didn't rank this first is because it does not directly contact the police.
3. Guardly
This app is available for all smart phones (yes, including blackberries). It's free to download, but does offer a premium service which can notify contacts, campus police, and 911. You can receive safety awareness alerts which you can respond to (for example if there is a shooter in your nearby area). It's possible to place an emergency call even if the phone is locked. With the premium subscription, it will even take photos in your distress time. There are also a variety of alert buttons such as theft, harassment, general assistance, and much more to let others know your exact crisis in an instant.
4. Send Help
It's a free app and probably the most simple of the bunch. It's a single button to alert your Facebook, Twitter, and all of the contacts you set with your location. This is only available for IOS phones.
5. Companion
This free app let's you map out your route and allows your contacts (companions) to see where you are, and when you've arrived safely. Companions don't need to download the app to see where you're at, but instead of notified with a GPS tracking link. If your phone is dropped, or you start to run, the app will confirm your safety. If there is no response in 15 seconds, all companions will be notified. The only drawback to this app is that once the destination is reached, it automatically sends out a "I'm safe" message. A person can't clarify if they're safely in the building.
6. SafeSnapp
Disclaimer: This app does not prevent situations! It merely provides evidence. With a press of the button, this app will take three consecutive pictures of your assailant along with your GPS and send them to your contact's email. It's a one time cost of 10 dollars for IOS and Google Play. It seems obvious why this is last on the list as it's pricey, doesn't prevent attacks, and may be a complete hit or miss depending if the victim can even get to an angle to get a good picture. However, if successful, it could be the deciding factor in a court case.