5. LoungeBuddy
If you have a long layover and don't want to spend it cooped up in the McDonald's food court, LoungeBuddy will let you know if any of the airline lounges in your airport are open to you. While they're often times reserved for only the ritziest or most frequent of flyers, many will let you in for free. If not, you can get a guest pass for around $25 or so, and use many of their free amenities inside.