Kayak is my favorite app for investigating flight schedules and costs. Its screen is easy to use and easy to read. I can watch fare changes by setting up an alert on a particular route. This helps me decide when to "pull the trigger' on the purchase of a flight. When I see the fare and schedule I like I can click right on the screen to go either the airline site or other travel sites to make my purchase. Kayak is FREE and available on iOS, Android and even the web. Kayak is a Universal app for iOS which means it looks perfect regardless of iPhone or iPad.
Trip Advisor is a great place to get reviews of great places to visit when traveling. Trip Advisor is very common internationally and I've used it in Cabo San Lucas, Mexico most recently. Check out Cabo Flyboard for one of the greatest water activities I've ever attempted. My cousin provides reviews from her home in Florence, Italy and I get to see lots of fun things to do there. Trip Advisor is very similar to Yelp and available on iOS, Android and the web, all for FREE
Gate Guru, which is owned by TripAdvisor, has an itinerary saver like TripIt but what Gare Guru does so well is show all of the Food, Shops and Service locations at each of the gates within an airport. It even includes ratings from other travelers too. There's nothing more frustrating than getting off a plane and wanting to find a shop or restaurant that has good quality. Gate Guru is available for iOS, Android, Windows Phone and is FREE.
Tripit is used to store my itineraries and travel plans. When I get an email confirmation of any type of travel I forward the email to Tripit and it is reformatted in consolidated and easy to read view of all travel plans for a particular trip. Tripit is FREE but has a PRO version too which can notify you of gate changes as well as fare changes for potential refunds. It is available on iOS, Android, Windows Phone 7, Blackberry and the web.
Please feel free to share your favorite Travel Apps too. I'd love to hear about them.